Unlock movement/edit of internal polygon layers in altium 19?
I guess you changed the layer within the layer stack manager. Usually this is a rather seldom step, which may cause some unexpected parameter shifts in the back-end of Altium. 

Hence, to be on the safe side I suggest the following procedure;


First I would add a new signal layer. 
Copy the polygons primitives, or even better use "Tool->Create from Primitives" to produce a fresh polygon on the new layer. 
Finally delete the old plane layer.