When you migrate your database from 6 to 8 your current DLU will port as is and you can work with licensing to rehome to the new virtual MAC. If you don’t have rights to upgrade then you move to 3x3 or CUWL. With 3x3 for 3 dollars per user you will get the rights to the software for major upgrades and DLU to turn on features like single number reach which is included. Your current DLU will port as is since those are perpetual.
You do the same for Uconn which also is applicable to 3x3..you will see options for 8.5/8.6 under CUCM-USR-LIC or CUWL-LIC-STD-K9 or CUWL-LIC-PRO-K9
Check the OG for more details
http://www.cisco.com/web/partners/sell/technology/ipc/uc_tech_readiness.html#~7
Thanks
Srini