yes i was thinking on exit trigger like replace walkable tile with solid wall, but not sure how many triggers are allowed per level- gbstudio limits them..
https://liftedpixelgames.itch.io/pxgba/devlog/1578655/034
added a "pass wall" actor type - you can set it to be solid after X frames, after X frames after being pass through, or after X amount of passes. set the idle animation to the state you want it to look like when the user can pass through it, and set the walk (or any other) animation state for when it's solid, then set the animation in the actor config :)