Generally speaking, I'd say to avoid that situation where you're purchasing fresh licenses for equipment that has gone End-of-Support, which it typically 5 to 6 years after the End-of-Sale date. EOST equipment is considered obsolete. You can generally purchase fresh licenses before the EOST date, but not after.
For example, the MS22/22P and MS42/42P switches all went End-of-Sale back in April 2014, and they can no longer be purchased. The licenses however, can be. I confirmed just now in CCW (Cisco Commerce Workspace) that the license SKUs are still in the system, and they do have "EOS" in the description but they are still orderable. I would not expect them to be orderable after EOST. But I suppose you could purchase a 5-year license today, which would last until 2023, even though the switches go EOST in 2021. That would perhaps keep Dashboard happy from a licensing perspective, but you'd have no support, could run into firmware upgrade issues, have no RMAs for a failed switch for example. And when there was a "License Problem" warning message come 2023, that would be almost 10 years after the equipment was announced EoS, and should be retired if it wasn't already.
If you wanted to keep some equipment around like in a lab (I have some Cisco switches that went EOST like 3 or 4 years ago) then I suppose you could keep them in a separate network and purchase the appropriate licensing before the EOST date.
Hope that helps. I'll double check on this and post back if I find out something different.
