Yes it is mandatory, with regard to the changes introduced are technical modifications visible in the commits on Github, I will add your question to the next AMA in the event that it is possible a brief non-technical explanation of all the numerous modifications visible in the various commits merged yesterday on Github. I seemed to understand that there will be no further releases before the fork scheduled for tomorrow. About the old working wallet you're right  the old wallet works and I can't tell you if it could work after the fork, but yesterday's announcement about the mandatory update was clear so I would advise against continuing to use the old wallet (especially after the fork of the upcoming 850k block, assuming the old wallet continues to sync). In the end it is just a matter of downloading and unpacking a zipped archive and launching the new qt, since there have been no splits or other problems a full resync or a "clean" installations are not needed this time, in other words it is just a matter of downloading, unpacking, launching the new qt, all very fast and immediate! 
For any need, do not hesitate to ask, here or where you prefer, of course. Thanks