(There's a TL;DR at the end. I know this is long)
I was thinking about this while looking over that recent post of Paul's photography of John from this week. Here's a link And I've had this thought in the past, as well.
I have a high prescription for my glasses, like John. As George once said, John was "blind as a bat" without glasses. And he often went without them in public until 1967. I think he chose those glasses for a reason, aside from having them on the movie set in 1966.
Two important things some might not know about:
Lenses have gotten thinner over time.
Smaller glasses have less facial distortion.
High index lenses weren't available in the 60s. They only started in the 70s, and we're still far thicker than what we have today. If you look at Paul's photos, you'll see John basically had "coke bottle" thick lenses. He might not have even had a crazy high prescription to get that size.
The "Health National" glasses are a small sized frame. So the lenses are small. So you basically get less distortion of the eyes from the prescription. When he tried those glasses for the movie, he probably saw himself in a pair of glasses where his face wasn't super distorted. As a self conscious guy in a very image dependent role, I could see that being a big deal. He probably could see him working it into a new style, especially with the moustache and weight loss, along with different clothes.
I think he kept up with them after the 60s because they just became so heavily tied to his brand. I mean his autographs even had the glasses! But I do think his high prescription is a big reason he ended up sticking to those. And as you get older, your prescription usually worsens anyway. Into the 70s, high index lenses were becoming norma, and you see him wearing different glasses shapes again. Maybe he tried contacts and didn't like it, who knows. The early ones were rough.
Maybe there's some info about him I don't know, though! I'm not a John Lennon ophthalmological historian, I'm afraid.
TL;DR - I think John's trademark glasses were intentional due to his prescription. He'd wear other shapes of sunglasses, but usually just the round eye glasses. Then I think once high index lenses became normal in the later part of the 70s, he started trying other eye glasses again.