Andy (and Sarah), In no way do I want this or my previous posts to sound disrespectful as I consider the two of you as some of the finest people in the chainsaw world.
With that being said, materially changing the classes just days before the race puts folks who have been prepping under the old rules at a serious disadvantage. I have 6 muffler modded but otherwise stock saws that at this point have no prayer of being competitive against woods ported saws from the best saw builders in the business.
Regarding race chains, I don't really think it makes much of a difference until you get into the open division. I saw lots of winners at last year's event running round ground chain. Response time and technique goes a long way.
Finally although the intent of the changes was to make things more fair, in reality unless you have one of the two or three top saw models in a class you really don't have a chance anyhow unless someone's saw won't start or they bog in the cut. IE the 50 cc class is almost certainly going to be won by a 550, 261, or Dolmar 51xx. If you show up with a 350 that has muffler mod, base gasket delete, and timing advanced you are still going to get spanked by a bone stock 550.
In the end it's your event and I'll respect any decision you make.