Windows Phone 7 is actually based on Windows Embedded CE 6.0 R3.
As with many of these older devices, OS upgrade was available via ROM chip replacement. A lot of folks took advantage of this with the Aero 1500 models, buying ROM chips with Pocket PC 2000 that were offered separately in addition to the pre-upgraded model
(Aero 1550
). As I've learned from my own experience buying a few Aero 2100 series models, a lot of people did the same thing with the 2100 series, buying a Pocket PC 2000 upgrade ROM chip specifically made for the 2100 series.