Find familiar says:
If you cast this spell while you already have a familiar, you instead cause it to adopt a new form. Choose one of the forms from the above list. Your familiar transforms into the chosen creature.
Upon transforming, it's stat block is replaced by the new creature. Does this remove any damage that existed on the previous form like most transformations that replace stats or does the damage carry over somehow?
Taking a new form "heals" it. Indeed, if your familiar is killed, that's how you get it back, re-summon its spirit with the spell to take form anew.
The spirit takes a form when it is summoned by the spell, so it would be a new body, whether it took the same form as before or a different one. By contrast, temporarily dismissing a familiar into its pocket dimension, and having it reappear, does not change the form, or heal it. Unless and until the familiar is re-summoned by the spell, it is the same form, and would be heal or be healed by spells or by resting as normal.
