Join Date: Feb 2005
Bikes: Trek 5500, Colnago C-50
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More information would certainly help.
You can probably do what you want to do so long as the rear shifter matches the number of cogs on the cassette or freewheel, and the shifter cable pull is fully compatible with the actuation ratio of the rear derailleur. Or if the new shifters are friction control, not indexed.
You will also need adapters on the downtube shifter bosses to use as cable housing stops.