This is already something of an edge case, and the intuitive answer would seem to be that the creature vanishes, right? But I'm interested in whether that is the case when certain items are involved. The relevant text for Ruby Weave Gem and Ring of Spell Storing are as follows:
Ring of Spell Storing: This ring stores spells cast into it, holding them until the attuned wearer uses them. The ring can store up to 5 levels worth of spells at a time. When found, it contains 1d6 - 1 levels of stored spells chosen by the DM. Any creature can cast a spell of 1st through 5th level into the ring by touching the ring as the spell is cast. The spell has no effect, other than to be stored in the ring. If the ring can't hold the spell, the spell is expended without effect. The level of the slot used to cast the spell determines how much space it uses. While wearing this ring, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell. The spell cast from the ring is no longer stored in it, freeing up space.
Ruby Weave Gem: ...When you finish a long rest, choose a spell from any class list. The spell you choose must be of a level you can cast. You know the chosen spell and can cast it with your spell slots of the appropriate level until the end of your next long rest.
The text for the spells Find Familiar and Find Steed (and its Greater variety,) only say explicitly that the creatures summoned disappear when the caster says so or when they drop to 0HP, so it seems RAW I could-- as a ranger-- have a (Greater) Steed, Familiar, and some other conjured thing from a non-ranger spell list; assuming none of the creatures hit 0 health and enough long rests have passed for me to have swapped the relevant spells out. My only issue with this interpretation is that it would seem to imply that casters that otherwise come by those spells naturally (looking at warlocks specifically,) would get to keep those creatures past swapping out pacts or spells, which seems somewhat... janked.
The alternative-- losing the creatures when you lose access to the spells-- would seem to mean that you just can't use the previously mentioned items (especially the ring,) to replicate those spells in any meaningful way.
Do I have the jist of this or am I missing something?
Paladins are prepared casters, not learned casters, which seems to reinforce the "creatures stay" option.