This is a known issue / customer surprise with a bow on top. At the time of this writing, the current best practice is to never upgrade your virtualized vCenter to 5.5 compatibility (virtual hardware 10). If you just need to change resources (i.e. add vCPU, Mem, etc.) then you can use PowerCLI for this task (see KB2061336).
To protect yourself from chicken and egg scenarios you may consider downgrading the vHardware. The only currently supported method is to revert to snapshot or use Standalone Converter (see KB1028019). However, some courageous users have had success manually editing the .vmx to virtualHW.version = "8" and then re-registering the VM. Again, that's not supported but is known to work.