Its not illegal to ask if something is possible in theory, so long as you dont get into discussion of details.
There are actually legitimate and legal reasons for wanting to change a phones ESN. For example, if you purchase a phone from someone and they never deactivated their account, then that person changes their phone number and you cannot get in touch with them, you may be unable to activate that phone even though you purchased it, legally. This actually happened to me once.
First off, discussing it is legal, discussing how, is not.
As for the rest....
No, no, no. Oh, and did I mention, No?
I studied up on this A LOT.
In some places changing an ESN is illegal, PERIOD. There is no extenuating circumstances, it is illegal. PERIOD. Missouri is one such place. It is illegal to change it under any circumstance. I don't even think they have an exception for carriers and manufacturers.
Technically what you are proposing as legal (not deactivating, etc..) is also illegal. A phone deactivated for non-payment or lost/stolen was deactivated for a reason. Swapping numbers to change it can get you charged with possession of stolen property but also fraud, since technically you are stealing from Sprint since the phone was never properly paid for (or early termination fee was never paid, either way, the phone is now dead legally).
And yes, even if the phone was legally purchased, unscrupulous and unknowing people can get the legal phone you just bought put onto the illegal list. You check with Sprint they say the esn is good, you buy the phone. The next day they call and cancel their account, which is still under contract and your phone now has a bad esn. Doesn't matter if if was legal when you bought it, changing the esn now to avoid that is fraud. You are breaking local and federal rules at that point. It doesn't matter if your intentions were good or not.
The newly proposed federal law about cloning will be specifically targeted (and meant to clear up confusion) at people doing this exact thing you are trying to say is legal.
Now, will you get busted?
Not likely, but it's still against the law.