They've only done this in the last week and I'm baffled too.
I tried my OEM COA Key and a Retail Upgrade DVD Key (I bought the DVD from Tesco on release in 2009). The Upgrade key is listed as Not Recognised.
Seeing as the isos have been freely available for years it's very annoying.
Maybe it's connected to the W10 giveaway.
In the meantime, keep several copies of your iso.
2 years old doesn't matter (as long as not corrupted), there haven't been any changes since then that are standalone downloadable.
It's all done by Windows update once the new W7 is installed.