I don't suppose this is a big deal to most so it is not surprising that Virtualmin does not allow one to change a private IP to a different private IP. For what it is worth, it would be useful for some hosting companies. In our case, we are so bloody old and started hosting with Windows that it was standard at that time to give out a dedicated IP to each domain account. We still do that even on our Linux boxes though I would imagine ARIN frowns on it now with so few IP4 addresses available. IP addresses we have by the thousands so we give everyone their own.
We have a Virtualmin Pro server that is acting oddly and some of the sites just stop working. We are not sure of the problem but our solution has been to move the sites to a newer machine. Ideally we would use a new and different IP address so we did not have to delete the old location first to free up the IP. Disabling does not delete the IP from the server so that is not an option. Using a shared IP first on the new machine might work, I have not tried that yet. (Using a new IP and then changing to a shared and then trying to change it to a different private IP does not work.)
Anyway, it would just be nice to be able to change a private IP to another private IP. I appreciate that it might never get to top of the list. We sure do love Virtualmin. Boo to cPanal, long live Virtualmin. :-)