I wrote a reply to this, but it looks like the DIS ate it

. I'll try again.
I don't think the effects on RD from this change will come from people who otherwise wouldn't have gone that early. I think it's pretty hard to pull a huge amount of people into the park early who otherwise wouldn't have, RD has always been a huge advantage, yet majority of people don't arrive until much later.
Instead, I think the change will come from people who already take advantage of RD. The biggest change will IMO come from the people in the know, who up until this point were maximizing FP+ by arriving at RD, riding SB, and using their FP+ in the afternoon/evening. I think that will be the group that shifts the most because of this.
It doesn't really work as nicely at DHS/Epcot because of the lack of attractions, or AK because let's face it, you really don't need much more than 3 there. At MK though (or for anyone hopping to MK later in the day, depending on how hopping rules work), there's lots of incentive to use FP+ as early as possible. For example, book FP+ at 9, 10 and 11. Ride SB until just before 10 when you use your first FP. After that ride SB again until just before 11, when you would use your 2nd and 3rd FP back to back. Preferably these are headliner type rides that you may not be able to grab a 4th+ FP+ for later in the day. After you exit the last ride, book another. These would be the secondary type rides that have gotten significant waits since the introduction of FP+.
At a park like MK, someone who takes full advantage of the system may be able to avoid lines for a good part of the day. It only really works if you are familiar with the park layouts so you can time your FP properly. Running around between rides, especially for the first 3 FP, kind of takes away the benefit of this. This still wouldn't be as significant as FP-, in terms of cutting down waits. Those days are gone. However, it seems likely that you would be able to grab secondary FP+ for a good part of the day, giving you walk ons all day long.
Anyways, long story short, it will be guests like the ones here on the DIS who screw up the SB lines because of this

. It likely won't encourage people to get their butts moving to get to the parks early, it is much more likely to be the people who already use RD to maximize their touring.