I will be surprised if the 3 day regional conventions go to two only because these are cash cows for the organization. I have worked in the accounting departments at disctrict conventions for many years and a convention of 7000 in attendance pulls in nearly 100,000 dollars.
Personally, I'd be shocked if this didn't happen in the next 10 years, tops. I'd bet closer to 3-5. Eliminating friday from the DC/RC probably won't change the amount contributed in any substantial way. People get into their routine as to how much they donate and they won't think about it unless they're told to donate more.
If they do have a drop in donations in the first year of the 2-day RC, the second year there will be a couple articles in the KM, and letters read. Some will be subtle about making sure we're "seeking first the kingdom" and sacrificing both our time and money to Jah. Then they'll have a letter read that will tell everyone the "per-publisher" cost of the convention, and at the end there will be a statement along the lines of: We know that many in the congregation will find it difficult to meet this cost, so those who have greater means may wish to contribute extra to cover the costs of the less fortunate. Of course almost no one in the congregation wants to view themselves as unable to cover the "modest" donation requirement, and most will donate well over the suggested amount for fear that they might be shamed by a shortfall. Not to mention the several KM articles that were read in the weeks before to set them up for it.
They really have to get rid of the friday from RCs, though. All the R/F I've heard talk about the change to only having 1-day assemblies have expressed confusion over why we have 3-day events in a venue that we have to pay for, but the stuff in the assembly hall that's free and clear is being reduced. It's just silly if their motivation was anything other than dollarbucks.