I guess this is the poster, it'll be tough to cover him and make it look decent. I couldn't find a pic of Jeremy in his gta gear where he's in a similar "side on" pose as Ryan has there (this is the only decent one I found), so swapping the top half only have would look very odd. Also there's like a nine or ten inch height difference between them so realism would be out the window..

So I think your best options are A) intentionally make it look bad, would be kinda funny that Jeremy is just slapped over dickhead, B) crop the poster, use a straight edge and a razor blade and cut the left side of the poster off, so no ryan but also no room for jeremy, C) befriend an extremely talented artist and get them to paint out ryan and paint in a hyper realistic jeremy, D) get it photoshopped and have a new version printed, would defeat the purpose of using the poster you already have but kinda the only option if you want it to look like it never altered