It depends on the make/model of the crank, and make/model of chainring.

I have a Shimano XT crankset on one bike with the 32T swapped for a 36T, and the 44T swapped for a 48T.
I needed to cut down the anti-chain-jam post behind the crankarm to enable it to fit.

You need to not only check the BCD of the chainrings, but also the tab of metal around the bolt hole may also extend toward the center of the crank to a different extent. The original 32T on that crank had a longer tab than the 36T that replaced it. In this case it worked, but the inverse combination of crank and chainring would not.
Here is a thread talking about the issue: http://www.socaltrailriders.org/inde...-issues.15866/