Changing drive letter for backed up Windows directory hierarchy
Thanks, that looks basically the same as the link that was previously posted which apparently only applies to "old format" backups.
We contacted MSP360 Support yesterday as David Gugick had suggested and they quickly responded. They conducted a test, backing up some files then moving them to another drive letter, editing the backup plan with the new drive letter, and continuing the backup. The pre-existing files were not uploaded again. This confirmed the de-duplication feature of the software where data is backed up at the block level and previously-seen blocks are not uploaded again.
Thanks to all who responded here, and kudos to the MSP360 support team for a very fast response and actually testing the feature to verify that it works.