Gavin's plan (as I understood from his posts) is to deploy a large-block version of the software now, but the change would be programmed so as to allow 20 MB blocks only starting in Q4/2016.

That way, hopefully most everybody will have time to upgrade to the new version and adapt their own programs to allow for 20 MB blocks -- including the major exchanges and services, and most miners.  If that happens, the critical date will pass without a ripple for those who have upgraded.  For the laggards and rebels still using the old version at that time, they will find that they cannot use the exchanges and services, or maybe even move their coins; until they upgrade too.