Question on Installing New Terrain Mesh

I've got FSGenesis now (10m mesh) and I want to try out the FS Dreamscapes ProMesh Hawaii (and maybe California too). The new mesh is 4.75m res. How should I handle this? Install directly over existing mesh? I want to keep areas that the higher res mesh doesn't cover. Can I just add this new mesh and will FSX know what to use (what is higher resolution)?
Thanks for your help!
-Jim
Thanks for your help!

-Jim