Upgrade from ATI 2013 on 3 PCs to ATI 2015 on 3 DIFFERENT PCs

I have ATI 2013 licence that allows me to pit it on 3 PCs. Did that and it worked fine.
Bought the Upgrade pkg of ATI 2015....it allows all 3 PCs to be upgraded to the 2015 version. Successfully
upgraded 1 or my 3 machines.

However, both PC#2 and PC#3 are about to be replaced with new PCs.
Can ATI2013 be Uninstalled from an old machine (that will no longer be used) and installed to a new PC so that it can subsequently be upgrade to ATI2015?

If yes, can you point me to the correct steps for Uninstalling ATI2013 from the PC that will be retired.
If not, why can's this be done?

Thx, GMK