Join Date: Oct 2007
Location: Québec, Québec, Canada
Bikes: Fuji Touring 64cm, Peugeot Canada PB-18 61cm and winter beaters
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I don't think it would work - you can't put a 8-speed cassette on a hub meant for a 7-speed cassette. Some conversions are easy - like 6-speed freewheel to 7-speed freewheel. Or 8-speed cassette to 9- or 10-speed cassette. But 7-speed cassette to 8-speed cassette typically requires a rear wheel replacement, which is not worth it.
If you don't like your shifters, just replace them and stay with a 7-speed drivetrain. You can still find 7-speed shifters, like the MC40
if you want to keep your existing brake levers and the ST-EF50
if you want an integrated unit. If you get the ST-EF50, they are made in two versions, one for v-brakes and the other for cantilever and caliper brakes.
Plenty of new bikes are still sold with 7-speed cassettes, it's a perfectly sound and reliable technology. There's little point in upgrading to 8-speed.