Want to be sure I am reading things correctly.
Have a customer that has 2800 and 3800 series routers in their network. They are all running 12.2 level of code and varying feature sets.
they are looking to upgrade to 15.x software levels, providing the hardware is supported.
If I am reading things correctly, the only time the software activation process comes into play is if the platform is an ISR G2 platform. If older platforms, Cisco is not going to be doing license key enforcements yet so the smae upgrade procedures are in affect as long as the routes are under SmartNet maintenance.
I have 871, 2821 and 2851 and they are all running 15.1(1)T. I'm testing 15.1(2)T as well.
As long as you have a valid Service Contract (which includes IOS download), you should be fine. The ISR G2, 3560E/3560X, 3750E/3750X have a new license mechanism where you download the base IOS and apply the license key to activate the feature set you need. G1's does not support this feature.
Before upgrading, you have to verify you have the correct amount of DRAM and Flash. If you look at 15.0 version, the requirements are pretty steep but if you look further to 15.1(1), you'll noticed that the DRAM/Flash requirement isn't that spectacular.
Hope this answers your question.