You can't, once the conversation has already started.
However, it is a highly requested feature (see for example here), so there is a decent possibility of it being added in the future, or a mod being created.
[Sept 2023] There's now a mod that makes it so the highest modifier in your party is always used. That's not quite the same as switching characters, but it's better than nothing.
The companions you can get are:
Side with the Grove: Everyone except Minthara, though that can be changed with mods
Side with the Goblins: You can recruit Minthara, but lose Wyll, Karlach, and Halsin. Gale also leaves if you fail a skill check.
Side with no one (enter the Shadow-Cursed Lands without picking a side): Everyone except Halsin
There used to be an exploit to get everyone but it has since been patched.
There's a few other places where companions can permanently die/leave
Act 1:
Lae'zel is killed by Shadowheart at night if you fail a skill-check
Karlach can be optionally killed as part of Wyll's storyline
Asterion can be optionally killed after he attempts to bite you
Gale leaves if you don't feed him magic items
Any companion can be sacrificed to Booooal
Act 2:
Gale can blow himself up
Shadowheart leaves if you free Nightsong without having her in the party
Jaheira dies if you kill Nightsong
Jaheira frequently dies on her own in the Moonrise Towers fight
Act 3:
Gale can still blow himself up
Astarion is sacrificed to Cazador if you don't free him or kill Cazador fast enough
Jaheira leaves if you don't save Minsc
Minsc dies if you don't have Jaheira in the party when you fight him
Best Answer
The spell used to summon Scratch is Find Familiar!
Generally speaking, there are two factors at play here: concentration, and spell uniqueness.
The interactions with Concentration are straightforward -- you can't concentrate on two spells, so two spells that summon creatures and require concentration can never exist simultaneously (at least, not from the same caster!).
This means you can't cast "Conjure Woodland Being" multiple times in order to fill the tactical grid with critters.
The other major interaction is Spell Uniqueness -- you can't benefit from the same spell more than once. This is why Scratch disappears -- you've replaced the previous "Find Familiar: Scratch" spell with "Find Familiar: Crab" (or what have you) -- same as if you cast the spell while your current familiar still exists.
I suppose one final variable is duration -- for instance, Spiritual Weapon lasts 10 turns, whereas Animate Dead lasts indefinitely (or until a long rest, at least). If the duration is too short, you'll find it harder to get overlapping summons.