I wanted to transfer my milanese loop band from my original Apple Watch to my new one last year but not even a jeweler could remove the band from the old watch!
So, it appears that the watch band release mechanism is frozen. Since you said it was an original Apple Watch, you probably hadn’t exercised it in years.
A lesson from this is that Apple Watch owners should exercise the mechanism on a regular basis–remove the current band and reattach it, even if no replacement is being done.
My parents ran into this as well—I can’t quite remember the details, but I wasn’t able to get one mechanism to budge at all.
I had one (homemade) band that was stuck, too, sometime within the past 6 months. I ended up using a Q tip with some WD-40 on it, rubbed it around the connector and voilà !
The band had not been moved at all for probably a couple of years, so maybe, just maybe, this issue was not limited to Apple’s own (Milanese) watch band.
The release button operated but the band would not slide out. Apple never said the bands had to removed frequently or they would not work. When I get the chance I’m going to spray it with BreakFree.