All Campy left shifters can operate a triple, but the the '07-'08 Centaur and Veloce shifters with the limited ability escape mechanism are the worst models Campy ever made.

If you just want to get by for one trip and can do without much top gear, you could indeed just switch to a 13-29 cassette, but you'll need a medium cage RD, unless you're willing to increase the chain length so it hangs loose in the little ring and several smallest cogs. If you're sure that you'll never shift to the 50/29, then you can just install the new cassette and change nothing else.

If you do change to a triple, all you need to know is the BB threading. Most likely it's English, unless you have certain brands of Italian frames. You will need a medium cage RD, new crank and BB and a new FD. A triple FD is a lot different than a double FD.