The fey theme continues at Level 7, where you gain advantage against being charmed or frightened. Eventually you learn to cast misty step almost at will and you can summon fey to do your bidding. Ranger – Swarmkeeper The other stand out for me is Piercer;any DEX-build martial with a Shortbow or Longbow and a Spear or Rapier gets a pretty significant damage boost. I can see Rangers or Kensei Monks especially getting alot of benefitout of this with their other bonuses ( Hunter's Mark or Swift Quiver for the Ranger, Kensei features and Ki-fuelled strike for the Kensei). This domain is pretty self-explanatory. Peace clerics oversee peace treaties and calm conflicts between warring nations. Their spell list is full of support options like sanctuary, beacon of hope, and greater restoration. The impact of permitting a subclass change at any point in time compared to one allowed at certain levels only largely depends on your play style. You may want to review "Know your players" (DMG 6). At one extreme would be if you had a table of "Optimizers", who "welcome any opportunity to demonstrate their characters' superiority." In this case, I would strongly recommend that you keep to a hard requirement of changing subclass only at subclass-defining level breaks, and that you impose the time and gold requirement of "Training time" as well (although not the quest). Anything less will invite these players to switch subclass solely for the mechanical advantages involved, and you will have things like them reasoning, "Now that I have magic item x or spell y, I get better synergy with subclass z, so I want to switch now." Granting a subclass change to one of them while disallowing it to another one is also likely to generate hard feelings.

Nothing in Crusher, Piercer, or Slasher says the damage has to be melee. I mean. Piercer would even work with the Ice Knife spell! Sorcerers are masters of using their internal power to modify their spells. You, too, have a small wellspring of this power that allows you to exert your will to twist the nature of your spells. You now have a small pool of sorcery points and two Metamagic options from the sorcerer class to spend them on. Piercer The College of Creation centers on the “Song of Creation,” the primeval harmonies that pre-exist the known world. These bards draw on this song using art, music, and poetry.

These three sections almost seem like different rules altogether for changing subclasses. I get that the two subsections, "Training Time" and "Sudden Change", shouldn't be mixed. However, the rule about only changing a subclass when gaining a new subclass feature is under the main section heading. This makes me believe this rule should be applied to both of the subsections. If not, then I feel like this should have been its own separate subsection as well. And all of the explanations make it clear that how it works is up to the DM. For instance, under " Training Time", it says (p. 8):

Waiting for a level up is how I would first express this to the player. If they felt that was too long to wait, I would of course see how we could work a transition even sooner. What about rogues? These are not rules in the traditional sense. These are suggestions a DM might use to facilitate a subclass change in their game. Under " Training Time", we see (emphasis mine):Akin to the Fey Touched feat mentioned earlier, a character might gain the Shadow Touched feat from being lost in the drab and ghastly landscape of the Shadowfell. In addition to improving one of your ability scores, you also gain the ability to cast invisibility, as well as another shadowy spell. Skill Expert This combat-centric domain centers on the dual premises of the restorative nature of sleep and the need to destroy the dark things that lurk in nightmares. These clerics gain proficiencies with martial weapons and heavy armor, and they can share their newfound darkvision with allies. Your channel divinity is an interesting buff that allows creatures close to you to gain temporary HP or end the charmed or frightened effect. The radius of this effect grows at higher levels. Druid – Circle of Stars

At higher levels, you and your party gain a boost to initiative rolls. You also can use your reaction to deal force damage when you or a nearby ally succeeds on intelligence, wisdom, or charisma saves. At Level 20 you gain truesight and a slew of benefits batting extraplanar beings. Ranger – Fey Wanderer The Circle of Stars draws on the natural magic of starlight. Understanding the patterns the stars make has opened your eyes to the magic of the skies. The theme of this class is largely based on navigating using the stars. You create a star map that can be a spellcasting focus and it grants you new cantrips. On the other hand, if you have a table of "Storytellers", a change at any time is fine, as long as it connects to the character and plot arcs you are creating. Allowing the change to coincide with an important dramatic event can even reinforce the player's connection to their character concept. In this case, you are likely using "Story-based advancement" (DMG 261) anyway, so the events that justify their "Sudden change" are also likely to be level breaks as well - although you can safely ignore whether or not the new level comes with a subclass feature. We start with a new artificer specialist: the armorer! As the name implies, this subclass is all about using your armor as a conduit for arcane magic. Using your smith’s tools, you can add a variety of buffs. At level three you can ignore armor strength requirements and use the armor as a spellcasting focus. You can also remove the armor as an action, and it can’t be removed from you against your will. It even replaces missing limbs!In either situation, I would work the subclass change at their next level up. In the first scenario, if they give me time to work in some plot, there is going to be some significant plot leading up to their next level up that will culminate in a change of subclass when they finally level up. There is an initial introductory paragraph under the section heading, and then two sections under subheadings. Textually / Structurally it would appear that the first paragraph contains the general rules, and then the two subheadings offer contrasting implementations. Thus, the design and layout of the rules imply that changing your subclass can occur only at level breaks when you gain a new subclass feature; that is a hard requirement for invoking this rule. Once the rule is in play, the GM then selects from two contrasting options for implementing it - will it cost resources ("Training time"; time, gold, a quest), or not ("Sudden change")?