If my Pact of the Chain warlock switches to Pact of the Tome, will it affect the form of my existing familiar from the Find Familiar spell?

Question

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?

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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.

Answer 1

You lose the ability to cast find familiar when you change pacts.

Pact of the Chain says:

You learn the find familiar spell and can cast it as a ritual.

So Pact of the Chain is the source of find familiar. If you switch from Pact of the Chain to Pact of the Tome, you lose the ability to cast find familiar familiar. So the question "what happens when I recast it after changing pacts" is moot since you cannot cast it at all.

Your familiar will hang around after you switch pacts, but once it drops to 0 hit points or you "dismiss it forever" it is gone for good, unless you somehow gain the ability to cast the spell again. However, it should be noted that it is entirely reasonable for a DM to rule that the services provided by the familiar are part-and-parcel to your current pact agreement, and changing terms would cause your familiar to vanish.

If you later find a way to recast find familiar, you may only choose one of the usual options, not the options provided by Pact of the Chain.

This part is quite simple: Pact of the Chain is what gives you the ability to choose one of the special forms. Without Pact of the Chain, you may not select one of the special forms. You always select from the available forms when you cast the spell, since the spell says:

You gain the service of a familiar, a spirit that takes an animal form you choose: bat, cat, crab, frog (toad), hawk, lizard, octopus, owl, poisonous snake, fish (quipper), rat, raven, sea horse, spider, or weasel.

The phrase "it reappears after you cast this spell again" is simply indicating that you must cast the spell again to reacquire the services of your familiar - it isn't bypassing the rest of the spell description.