...my usual C --> users is actually D -->users, and that whole thing is kind of a bitch to set up IIRC, and I don't want it to get messed up.
This is a non-issue, and very easy to manage. I do it frequently.
When I recover an OS image, it's in plain text. I only encrypt it after updating the OS with changes I want permanent, and save a new image for the next time I revert. Since there is no encryption of the OS at that time, there is also no pre-boot authorization that would allow other partitions to mount automatically.
Since my User directory is on my D partition, which is encrypted, the OS is unable to read it before I mount that partition. Later, once the OS is encrypted again, D can be auto-mounted with the pre-boot authorization. Until then, there are a couple complaints that D does not exist, but it is not a problem. I simply ignore these messages, mount D, and everything everything works fine. I can't access my personal data, but drive letters can be changed, and other maintenance can be performed normally.
In any case, it is a simple matter to change drive letters through Disk Management, once you have copied all your data around. Until you do, the OS will still operate just fine. There is no need to clone per se, a simple copy will work fine, followed by a drive letter change.