Well, now, guess what I've just done - stuck 700c wheels in place of 27" jobbies on my old 531, bought c. 1988 and barely used since...
Bike has mudguard eyes and I found it was a little tight running 27" wheels and tyres, plus wheels were old, had a freewheel not cassette and frankly, 27" stuff is hard to find.
Saw some second hand handbuilt MA3 on 105 combos on CTC site, and £45 later...
Lessons: 1. 27" wheels diameter is 630 mm; 700c is 622mm so you need 4mm of drop left in your brakes - if not got that, plenty of long drop ones out there, eg Spa cycles do the Alhonga dual pivots
2. "Cold setting" steel frames: spoke to a chap at Condor, and he said "don't bother" if it's 126mm spacing and you're going to 130 (modern road spacing) - and he was right, it's easy just to pull the back end apart
3. Old gears work OK with modern cassettes (I have Shimano 600EX from '80s) - but I fitted a new chain just in case
I should have gone metric new wheelie years ago, they transform the bike...