If part number changes, how will the customer know there is now a new part?
Every online part retailer worth their salt has some indirect access to car vendor's part database that, surprise surprise, includes part substitutions. Had you ordered any car parts online, especially ones that fail due to bad design and get redesigns/upgrades, you'd have seen it. This ignition switch part is rather unusual in this respect - a redesign was done without retiring the old P/N and having a "new and improved" P/N as a designated replacement.
On some cars, I've seen parts go through 2 or 3 rounds of such upgrade cycles. When I eventually pulled the failed part off the car and went to buy a replacement, there was a substitute part #1. That substitute was substituted with #2. Eventually, that one had substitute #3, which was orderable.