With the way Server Templates work – there isn’t actually a benefit to changing it, once the domain is created.
The Server Template is only used during the process of creating a domain.
Account Plans are different though – Account Plans are active, and in use perpetually… so there’s a way to modify that at any time if need be.
It’s possible to mass-change the Server Template using the Virtualmin API. You can do that by looping over all the domains using a given template, and then calling the modify-domain function and passing in your desired template.
However, there’s no benefit to doing that, as changing the Server Template after the domain is created doesn’t change anything in the domain.
There are some cases when the template is still used after domain creation: E.g. when you turn features off/on, like when recreating a DNS zone by toggling that feature. So I do consider having the correct template assigned to domains as noteworthy.