I hear you on the sizing weirdness—my last place was built in the 70s and not a single window matched anything in the catalogs. Ended up having to custom order a couple panes, which wasn’t cheap, but still beat full replacements. I’m with you: if the frames aren’t rotted and the sashes are still decent, just swapping out the IGUs is usually the way to go.
That said, I’ve had mixed luck with matching coatings. One room ended up with two windows that look slightly different in certain light. Not a big deal for me, but it’d probably bug someone pickier. Also, some of those “fix foggy glass in place” companies promise miracles but… let’s just say I wouldn’t waste my money there. If you’re already pulling stops and getting your hands dirty, might as well do it right.
Curious if anyone’s found a good source for odd-sized IGUs that doesn’t take months to deliver? That’s always been my biggest headache.