Hello everyone,
i have my pi 3 b+ working fine with retropie!
I´ve upgraded my retropie installation on the pi 3 to raspbian stretch with the following instructions:
https://retropie.org.uk/forum/topic/16145/retropie-upgrading-raspbian-jessie-to-stretch
after that, i put the sd-card in the new pi3 b+ and it looks like everything is working fine!