Are you planning on using Win7’s “convert GPT” utility? That should work (I did it on one of my systems years ago). Probably no need to disable CSM but it might be a good idea to do so to make sure you are installing Manjaro in (U)EFI mode.
When booted from the Live USB or DVD run
[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS
in a Terminal to check which mode you’ve booted with.
Note: I use GPT for both Legacy and (U)EFI systems and have done so for some time with mo issues.