Hello,
I was wondering if I change from a Campagnolo Chorus 12-27 to a 12-29 cassette, will the current chain length still be ok for the 12-29 or will I have to ad a link with the new chain ?
I will use the rear wheel with a SR11 compact and standard crank.
Thanks for any input.

Unlikely.

Unless the chain fitted was cut super short. Normally I cut my chain longest possible with out rubbing on R/Derailleur when in 39/11. This allows cassette swapping from 23 to 25.

I did cut the chain according to the Campagnolo manual that comes with the chain.

I think you will be fine. I run a 11-29, I buy a 11-25 and a 12-29 and make a hybrid. You size the chain based on the smallest cog. You will need to adjust what they call the "b screw" when going with the 29. The instructions are in the RD manual.

We can all theorize all we want, but what help is it? You'll only know for sure when you install the new cassette and try it. Then either add the links or buy a new chain if you need to. You obviously know how to fit a chain properly, but the definitive test for the old chain on the larger cassette would be the "big-big" method. I don't know what Campy instructions call for, but I'd use the big-big in this situation as it gives you the minimum length you can use safely.

My speculation: if your campy instructions are similar to the "small small" way of fitting a chain (which gives you the longest chain you can get away with), I'd guess your old chain will be long enough for the larger cassette. If your Campy instructions are the "big big" method, I don't think your old chain will be long enough.

As far I can remember I used the "Small Small" way. I will look it up when I come home since I still have the instructions at home. I did order a new chain with the cassette together to make sure I have what I need, should the one I'm using now be to short. It's always good to have spare components anyway.

And a chain will certainly be used, never a waste to buy one just in case you need it.