Les,
If you've paid to reserve seats and they change the plane they should give you comparible seating. Problem is the layout on a 76 and 75 is different (2-2-2 business, 2-3-2 ecc on 76, and 2-2 business 3-3 ecc on 75) so you'll be moved around a bit depending on what plane you get. you'll also have 2 aisles on a 76 compared to 1 on the 75.
As the current 76 has the original 3 class layout the early birds should get the old PE seats with 34" pitch as well. If they get another 76 that could change.