How to inserty custom background or second plan footage using dynamic mask in After Effects?
Something like that is solved with "Rotoscoping". After Effects has several features that make this process easier. F.e. the "Roto Brush", you can find a tutorial about that here:

                
            
https://helpx.adobe.com/after-effects/using/roto-brush-refine-matte.html

Then you will have to track the translation and rotation of the sign as its carried around. When you have that you can add a layer that follows the movement of the sign that holds your new content.