ArcGIS Desktop 10.0 machines should be able to access a 10.2 LM with no problem.
If if you are trying to move or combined licenses from two different LM into on 10.2, that is possible if they are all licensed for 10.2 or above, or if they are on current maintenance. In that case you can follow Jayanta Poddar advice. If they are not licensed for 10.2, that will not work.
Keep in mind that if you have a total of, let's say three 10.2 licenses that are on maintenance and previous were 10.0, your myEsri page may look like you have three at 10.0 and three at 10.2, but it is really an "or". That is, you only have the quantity that you have had on maintenance, not double. That is always a bit confusing on that page.
edit. For license manager 10.2 .. Quick Start Guides