One doubt, today the FTD is all configured via FDM, if I put FMCv it will lose the configuration, so I back the FDM backup image would be possible via the Mangement interface? Would it always be online?
If converting from FDM managed to FMC, you could just wipe the config by temporarily changing the firewall mode from routed to transparent and then back to routed. More information here. I don't believe it resets the management interface IP address (I don't have an FTD to check right now), but you may wish to confirm before doing it on a remote FTD.
If you follow those instructions to change from routed to transparent and back to routed mode, this removes the configuration including the data interfaces configuration. The management interface configuration (IP address/gateway) is NOT reset. I tested in the lab and completed the entire process by connecting to the management IP using SSH.
After the configuration has been reset, you can then configure the FMC as the manager.