The skeleton is assigned per UAnimInstance class. (which your blueprint is derived from) If you want to change the skeleton asset association, you need to create a new class. This means that you have to make a main animation blueprint and create a child blueprint from it. (right click animbp and Create child blueprint class) Then change the skeleton association in the child blueprint Class Settings. (close the blueprint and open it again after doing this)
You will run into a problem after doing this, which is that the animation overrides are bugged and it will not allow you to select animations from your new skeleton. The only way to get around this is to create sequence variables in your main blueprint for every playback node, then update your variable assignments in the child blueprint. The only downside here is that some nodes, like random sequence player, does not support variable inputs.