Changing Cassette = Longer Chain?
 

I'm thinking of going from an 11-23 cassette to a 12-27 for better climbing on my double. Will I need to get a longer chain as a result?

Yes, the chain will need to be a couple of links longer, but you should replace the chain with the cassette switch, so it shouldn't be a huge deal to do both at the same time.

I sometimes change from a 12-23 to a 12-27. I have a chain for each. 4 extra teeth means a couple of extra links. The shorter chain would work fine except for the 53-27 and 53-24 combos, but for the price of a chain why risk any damage?

As an aside, depending on how your derailleur is set up, you may need to adjust the B-screw. A standard Shimano short cage derailleur (mine is 105) works fine with the 27-tooth cog.

So here is a thought...
I was running a 21-11 (until my crappy bike store lost my cassette, and replaced it with a 23-11, luckly for them I only had 25mile on the new chain), but long story short, I never added any links to it and it prevents me from running 53-23 and 39-11 combinations (which we all know are bad), so if you want to make sure that you will not run these combinations there is a solution

Maybe, maybe not. It depends how long your chain was from the start. There are two ways to measure a chain. You can measure a chain so it its long enough to work in the big/big combo. If your old chain was measured this way it WILL NOT work. The second way to measure is in the small/small combo. When measuring this way make the chain as long as you can but not so long it sags or rubs on the bottom of the rear derailleur cage. If your chain was measured this way it MAY work. Throw the cassettte on. If it works it works. If it doesnt it.......