Replacing grip shifters
 

Our Univega tandem has grip shifters which I just don't like. I think it's more of a preference thing, but they're just not for me.

Is it very difficult to replace them with thumb shifters?

Not difficult at all. Just match the new shifters with the derailleurs. (eg. Shimano 9-spd derailleur = Shimano 9-spd shifter, SRAM 1:1 ratio derailleur = SRAM 1:1 ratio shifter.)

I've replaced gripshifts with SRAM triggershifters on two bikes.
SRAM shifters with numbers (SX5, X-7, X-9, etc) work with SRAM rear derailleurs and SRAM shifters with names (Attack, Rocket, etc) work with Shimano RD's.
Shimano triggershifters work with Shimano RD's.

As mentioned above, the rear derailleur and rear shifter cable pull ratios must match.

It's a hell of a lot easier than replacing gripshifts with other gripshifts, I'll tell you that much.

Thanks for the info. I'll try to tackle this this weekend.

+2 I hate grip shifters. You will find trigger shifters to be easier to install and maintain (and use).

I agree with anything bad people have to say about grip shifters.