I've moved this thread to MV support. Please be sure to post your threads in the correct forum next time. Thank you.
Plugin releases is for presenting plugins you've written yourself, an you don't even need a plugin for this.
It depends on what exactly you want, there are several ways to do this (and yes, you could do it with a plugin, it's just not the only way)
If you make the transformation a state, then you can have that state add new skills and/or seal old skills. That methode has no sprite change for the actor, but it could be added to any visual transformation you might want.
If you don't have a special battle plugin, the easiest way would be to have the transformation skill call a common event and then change whatever you want in that common event.