There are three possibilities with regards to same-day FP/FP+:
1) There won't be any, although guests with FP+ scheduled already can change them. This is exceedingly doubtful, since it excludes a large fraction of guests, and allowing at least FP+ scheduling the same day is no different that allowing it the day before, really.
2) There will be some, but it is only what is left over after pre-books. So, given predicted worst-case numbers, E-tickets won't have any available by the time that day arrives.
3) There will be some, a combination of leftovers plus a number held in reserve. Day guests fight over them, as there will be far less than under the current system. Ahh, but everyone assumes that the same # of FP's being distributed under the current system will be distributed under the FP+ system. i contend that with the live data, they can actually fit more FP+ in. Also, the wasted FP's under the current system will also be utilized in teh FP+ system. I believe these will be the two factors that increase the number for FP+ for attractions...efficiency with live data and actuall utilization of wasted FP's as they can go back into the system the moment they expire.
2 or 3 is likely, but no idea which. Until the system is completely rolled out to everyone, we likely won't know.
And then it still remains to be seen that even if 2/3 happens, whether those who already pre-booked 3 FP+ can get more, or whether day guests will also be limited to 3 regardless.