You say you're interested in collecting all the sprint PM2's and you traded a unused 2011 20CP for a 2016 M4 because it had some cosmetic issues that others mentioned are somewhat typical of that particular model, and ask what would I do. I would have kept the 20CP, and thanked Spyderco for stepping up with such gracious offer !
How many 20CP models are available for sale today, next week, next year vs. M4's ...?
Trading such drastically differing knives indicates this was probably more about collectables than performance (as you mentioned). Chances are that 20CP will go into an employee's collection, or get notched and resold as a second forever decreasing it's collectivity value factor :-(
From a collectables perspective, I would plead a second thoughts attempt for a trade-back before it's too late. Those character marks that pop out under magnification could be moderated (massaged and smoothed if they really bother you) or simply left as character marks related to the history of the time & model.
What's important is how you feel about your new Jade M4, NOT what any of us feel ;-)
Appreciate your follow-up here, and once again a great example of Spyderco taking care of a customer (well ... not really even a Spyderco customer as relates to that specific knife, remember we are taking about a second-hand or more previous owners knife from a product run 5 years ago).
Thanks Spyderco ;-)
Regards,