Hi Dan.
Storage units don't come into play during restores so that's not an issue here. The main point is that NetBackup won't mount a tape unless the media density matches the drive type, i.e. HCART to HCART or HCART2 to HCART2.
However, you can change the drive easily "on the fly." Prior to the restore, just go into the properties of one of your drives and change it from HCART2 to HCART. When the restore is complete, change it back and it will be ready for the night's backups.
It really is as simple as it sounds.