@ShadowDragon yes, you're wrong ;-)
@tekiro what you want depends on your battlesystem.
if you're using the default battlesystem without any plugin, then you don't need a plugin for a transformation - you can use pure eventing to either change the actor, or change the skills of the actor in any way. In fact I can think of several different ways to do what you want by event commands, AS LONG AS you're using default battle system.
If you are using any battlesystem plugins, then it completely depends on which plugins you're using. Several battle plugins change actor data on use. As soon as the actor data is changed, any summoning or transformation mid-battle might destroy those specific data changes, resulting in any error during battle.
To prevent this, you'll need the summoning plugin to replace that changed actor data for that battlesystem.
That means that any summoning plugin only works for those battlesystem plugins that it is intended to work with.
So that transformation plugin linked above will only work if you're focusing on yanfly's plugins for your battlesystem - if you have a different one (for example moghunters) then it will most likely never work.
So before anyone can help you, you need to tell us what battlesystem you're using, only then we can link you to a compatible summoning/transformation plugin.