Chain length and cassette tooth count relationship A little context first: I'm going to be riding in the French Alps and plan to tackle some of the classic climbs (Alpe D'Huez, Galibier, etc) next month - I currently have a Campagnolo Athena 50/34 (compact) chainset paired to an Athena 11 speed 11-27 cassette on the rear. None of the drive-train components are anywhere near worn enough for that to be an issue. Additionally, the rear derailleur cage is able to deal with the full range of the 11-29 cassette so this is also not an issue.

What I am considering is swapping out the 11-27 cassette with an 11-29 to give myself a little extra in terms of 'safety gears' - better to be overgeared than undergeared in my opinion! 

My question is this: will I have to replace the chain when replacing the cassette, for example due to chain length? 

My reasoning is that since I will only ever use the 29 tooth on the cassette with the 34 tooth chainring, that chain length should not be an issue. As above, the chain is not in need of replacement due to wear, so that is not part of the consideration. Perhaps I'm missing something?
I agree with @ Nik that it should not be an issue. You can verify  it by shifting the chain to the big ring in the front and the large cog in the rear. With your hands fold the chain over  itself to simulate it being two links shorter. Observe the rear derailleur. You should see two bends in the chains as it passes around the jockey wheels. If the chain passes thru the jockey wheels in a straight line the chain will be too short.