I recommend pre-duplicating the slice in the nanobsd new installation images so the step can be skipped by the user for the first upgrade.
This is exactly what is already done on the Nano images. The second slice is merely a duplicate of the first done beforehand.
I've never had a problem running a nano upgrade, not recently anyhow. Depending on the version you started with, there may have been bugs in the code at that point that didn't allow a working upgrade, so a reflash may have been necessary.