I'm currently playing a Warlock, and at 3rd level, I picked Pact of the Chain as my Pact Boon. I then, using one of the special forms provided by Pact of the Chain, summoned a Sprite familiar:
When you cast the spell, you can choose one of the normal forms for your familiar or one of the following special forms: imp, pseudodragon, quasit, or sprite.
At 4th level, I plan to use the optional rule Eldritch Versatility (TCoE, p. 70) to change from Pact of the Chain to Pact of the Tome.
Would that affect my existing familiar's form? Or would my familiar stay as a sprite?
Additionally, if my familiar were to drop to 0 hit points and I cast Find Familiar again, would the familiar stay as a sprite?
The resummoning section of the Find Familiar spell description does not state any change in the form of your familiar when revived:
When the familiar drops to 0 hit points, it disappears, leaving behind no physical form. It reappears after you cast this spell again.
Only when casting it with a familiar already active does the spell description state any change to form:
If you cast this spell while you already have a familiar, you instead cause it to adopt a new form.