So, the limit is defined in main.h, line 42.
https://github.com/bitcoin/bitcoin/blob/5505a1b13f75af9f0f6421b42d97c06e079db345/src/main.h#L42

And the test is done at main.cpp at line 727.
https://github.com/bitcoin/bitcoin/blob/3222802ea11053f0dd69c99fc2f33edff554dc17/src/main.cpp#L727

So this looks like a very simple change. 

Is there any way for someone to play sillybuggers right around the transition period?  With the main chain and other chains at different heights, can the client ever be in a state where it's willing to accept larger blocks in one chain, or unwilling to accept larger blocks in one chain, due to the block height of some different chain?