my 620 is staying with 27" wheels. a frame is strongest when at it's original configuration, and i don't need the grief. changing everything to modern components kind of defeats buying a vintage bicycle, if i need 'modern' then my other bike has brifters and 700c wheels.
my vintage/classic bicycles give me the old skool experience...and those old 27" wheels/tires are rather comfy.
Changing to 700c has nothing to do with modernizing the bike. My '83 720 wears a set of 700c wheels with early Specialized sealed bearing hubs. Now I have a nice, very low maintenance hub combined with an increased tire selection. Nothing else was updated, I just wanted a broader selection of tires, and the sealed bearing hubs are hardly what one would call a downgrade.
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My bikes: '81 Trek 957, '83 Trek 720, '84 Trek 770, '85 Centurion Cinelli