If Disney did start to roll this out in late July - it would take months before it trickled down to all customers.
Initially there will be a time when both FP systems would be working at the same time without much difference from the current model. Most people wouldn't even notice the difference.
One wrinkle that could cause some concern for people right away on the slow roll out - would be the new [FP+ only] opportunities.
So let's say that I am relieved that I don't have to mess with FP+, and I can tour like I am accustomed to doing. The rub is going to come with those with FP+ getting in ahead of me on things like Enchanted Tales with Belle, Jedi Academy, Possibly Fantasmic (if they move away from paper tickets in the current test to only FP+), QS lunches, or even Pirates of the Caribbean (I don't know maybe that would be an issue).
The bigger issue would be when the number of people on FP+ increases past an even balance and they start grabbing the headliner rides days in advance, and the regular FP only has limited options. I would think that would be several months after the roll out.
This is all based on the assumption that the apps will work. Which right now, the entire software package drives me crazy (my disney experience, DVC home page, and the ipad all have different information for my upcoming trip).
Even though the FP+ option will be a beautiful thing for some people, it will cause some irritation for people who are looking forward to their trips, have spent lots of money, and will run into new problems not expected.
I think it is best to be prepared for all possibilities.
But nothing really matters until those magicbands are released and that website
is working.