I've been advised not to change my ID letter after 45 days. What is the big deal? Since the letter isn't sent to the IRS, how would they ever know that we swapped it for another one later?
You received good advice. Changing your letter or swapping it for another after the deadline can be considered tax fraud. One attorney that helped a client do exactly this decided to turn state's evidence to keep from going to jail. The exchange was disallowed, the exchanger was assessed an 80% penalty, and severely reprimanded by the Court. You have 45 days to select potential replacement property to identify. After that, your choices are set and cannot legally be altered.