Just to be clear... I didn't lose my seat assignment, but rather the type of seat. 15D in original (2nd exit row) turns into 15D in Oasis (non-reclining main cabin row before exit row).
I think I know the answer, AA does not even try to re-assign like seats to like seats in an equipment swap.
For example, I know not to book the "B" aisle seat in an MD-80 because it can change to a "B" middle seat on any other aircraft.
1A on E175/190 can disappear when an alternate version is substituted.
So now, on a 738, FC rows 5-6, MCE bulkhead row 7 and exit rows 14-15 can all disappear on an Oasis substitution and be left with random seats.
It just seems ridiculous to not attempt to keep customers in a similar seat to what was booked, especially when there is an obvious equivalent seat.
And yes, the Oasis aircraft are ridiculous.