I have an ARM64 aarch64 instance running with Debian 11 and Virtualmin 7. There were no problems with the install.
I found a few Virtualmin 7 packages, using apt-cache pkgnames | grep virtualmin and apt-cache madison, that had multiple Virtualmin 7 packages, such as virtualmin-config and virtualmin-lemp-stack-minimal.
Downgrading down and back up again, for example using apt install virtualmin-lemp-stack-minimal=7.024-1, did not cause an issue.
I need to find a virtualmin package to downgrade to with architectural dependencies, if that was the cause of the upgrade issue with the previous install. Since Virtualmin 7 is new, there may not be any.
As of right now, I am left with a puzzle why I could do a previous install of Virtualmin on aarch64 and why Virtualmin recommended upgrades failed. The previous distribution tried was Rocky, not Debian and was probably Virtualmin 6. My recollection is that there was an architecture mismatch.
I will leave the instance running. Or maybe snapshot it, delete it and resurrect it now and then as upgrades become available.
If a Virtualmin install works but package upgrades do not work then this points to an issue that should be easily fixed.