Hi! The plugin allows you to do “Animation Retargeting” (see Animation Retargeting in Unreal Engine | Unreal Engine 5.3 Documentation, Using Retargeted Animations in Unreal Engine | Unreal Engine 5.3 Documentation, https://docs.unrealengine.com/latest/INT/Engine/Animation/RetargetingDifferentSkeletons/) of Mixamo assets to Unreal Engine 4 ones, i.e. it allows you to share animations between Mixamo and UE4 characters. It doesn’t change bone names or other stuff (they are not needed for Animation Retargeting) and doesn’t help to share animations between different Mixamo characters (only Mixamo <-> UE4). Your original animations on the retargeted character will not be affected and will continue to work as before. After the retargeting, you must use the standard UE4 pipeline to work with the retargeted assets (see links before).
I suspect the problem is “before” using our plugin (our plugin will allow you to apply UE4 animations to your new Mixamo character for example - or apply Mixamo animations to your UE4 characters). Does your new Mixamo character (without modifying it with our plugin) works with your old animations?