Hello LorenzoJ ,
Welcome to our forum, hope you enjoy the contacts !
I wonder if it would be reasonably easy to just change one player vessel for another.
The short answer is NO, you can´t !
The vessel that got WP´s atached can´t be replaced; even removing it is probably damaging that mission !
There are many experts here, who have all the answers. But that is to simple !
I would just advise you, to read the ME-manual first and / or start making simple missions for yourself.
That way you would learn a lot about the ME-editor and its functions !
vr. gr. / kind regards Erik