Should be fine, unless you have manually modified things like mydestination or myhostname in the Postfix configuration (which is generally a bad idea, better to actually name the system what you want to use for those things).
There are many ways you could lock in the hostname of the system and make it harder to change, but nothing in the default Virtualmin configuration does so.
The default domain (if you have one) will also probably need to renamed (if you use it). This is another reason for me to not like the default domain. Now that you mention it, I’m not sure about the implications of that.
@Jamie@Ilia what happens with the default domain (the one created automatically based on the system hostname) when the system hostname is changed?
Would this be easier to create a new virtual server with the name I want (abc.domain.com) then somehow move/migrate my mail server from the default one to the new one?
RDNS does not need to match the hostname of your system. RDNS needs to exist, and the name it resolves to needs to resolve back to your IP. It does not need to match any particular name being hosted on the system.