Pact of the Chain says (bold text mine):
You learn the find familiar spell and can cast it (...)
When you cast the spell, (...) choose one of the following special forms: imp, pseudodragon, quasit, sprite
The Find Familiar spell says:
You gain the service of (...) a spirit (...), the familiar has the statistics of the chosen form though it is a celestial, fiend or fey (your choice)
If you cast this spell while you have a familiar, you instead cause it to adopt a new form. (...) Your familiar transforms into the new creature.
Can a Pact of the Chain Warlock that has a celestial familiar cast the spell again and have assume the imp shape? Would it be considered a celestial?