Steps to reproduce:

Install 6.0.0-OD-2015.48.1-0006
Import data from Cloud (optional)
Attempt to upgrade using 5.10.0 installer

Expected result:
Upgrade successful (as build number for 5.10.0 is higher than for 6.0.0-OD...)

Actual result:
"You can't use the upgrader to downgrade Confluence" error.