Good plan, I did that. In the end it was deceptively easy to persuade my DS1812+ it is really a DS1813+ instead. I didn't bother to downgrade, just installed a blank old SSD, did the file exchange and booted it up ... tada! DS1813+ reporting for duty!   To summarise for those who follow (keeping in ind I kept mine simple - one drive; last DS1812+ OS installed; no important data to lose - even if this works, your DSM version is going to be out of date so I'd be VERY cautious about