Are you saying 'airable in HD' because they protected the wider aspect ratio while shooting? Cuz they didn't abandon film and switch to hd till the last year it was on...
You're mixing up two different things, HD and widescreen.
They're not the same thing.
You can have widescreen that's Standard Def, and High Def that's not widescreen.
Enterprise was done in widescreen AND high def all along.
The switch to video taping on Enterprise did nothing to change the picture quality.
You can get High Def recordings off films too, not just video tape, as what was done with TOS proves.
Enterprise was running in HD all along, even when it was being filmed.
It was also being presented in widescreen, which analog TV stations ran as letterboxed.
One exception to this was, I understand, in England, where ENTERPRISE was run in 4x3/12x9 rather than 16x9 (widescreen).
They ran it with the sides chopped off.
My local UPN affiliate actually did that with one episode during the final season. When they did the second showing of that episode later in the week, it was back to widescreen/letterbox.
I have no idea what they did the night it ran 4x3, and apparently nobody at the station did either that night.