What version of RetroPie do you have currently installed ? If you used an image version older than 4.6, then you'll need to install from a new 4.6 image on the new system.

To transfer your existing files, backup your ROM/BIOS/Config files from the 3A+ installation, then restore them on the new installation.

@saccublenda as cunning as a rock that fella!

@mitu Thank you!

I have not done any of this in a while and I noobed out on the basics. Much appreciated.