Be careful when issuing the command configure manager delete. This command in itself does not do anything to the configuration of the device. you will still have configuration on it and it will still pass traffic. When you remove the device from management interface configuration, as seen in the FMC is removed and therefore all configuration that references interface names will also be removed. So, work that will need to be done once you re-add the device is to associate the interfaces with their respective security zones and security groups, configure all static routing (I am unsure of dynamic routing but assume you have to reconfigure this also), as well as any VPN configuration that references the device interfaces. You will also need to associate the ACP policy, NAT policy, flexconfig policy, Health policy, etc. with the device.
When adding it back the FMC will do a discovery and "should" fetch the interface configuration that is on the FTD device so you should not need to configure this again, but make sure to take note of this configuration as well as any other configuration that I mentioned above and possible more.
In summary, I suggest that if you do remove the device from management with the intention of adding it back, be prepared to have to manually configure the device from scratch, with the exception of policies. So be sure to make note of preshared keys, routing, and security zone / security group associations at the very least.
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