replace LTM and gtm discovery

It is depend how did you deploy new platform. Was it UCS or HA sync? if yes, then there should be no reason to run big-ip add.

if it is fresh install then yes.

You can easily identify if bigip_add is required. On the gtm run "tmsh show gtm iquery " if the status is not "connected" then you need to run it.

As alternative, you can "tail -f /var/log/gtm| grep " to see the error. If you see cert error, then you need to run bigip_add