Well, the segwit code is actually the same as used by core and has had all the rigorous testing already performed!  The new development will be the extension to larger blocks.  I would assume this is just changing a hard coded value somewhere, if that was the case then it shouldn't be that difficult to code, as most emphasis would be on tweaking existing test procedures to consider the new block size).  Having said that, it probably isn't coded in this way (?) and there are perhaps indirectly affected processes, nothing it seems, is ever easy when programming haha.