I fly the 738s every week and see this happen all the time when they switch from an older 738 to a newer one. I never book row 7 for this reason and stick with 8 to avoid the issue.

The older layout 738s start with row 7 and have 5 rows of MCE (rows 7-9 & exit rows 14-15). When AA started taking delivery of the "new" 738s (with in seat IFE) they added more MCE seating (rows 8-15) which reduced one row.

Now with that being said, I have recently been on some very new 738s that AA has reverted back to the old layout (with row 7) and I've hard that they are going to be reverting all of them back to that layout as they were not getting the revenue they were expecting to from the additional MCE seats that they added.

If you are concerned about it, one way to try to tell what it will be is to check the tail number at t-24hrs on FlightStats or something similar. I've found that any 738 tail numbers that are NxxxAN or any tail number before N939NN will have row 7. Anything from N940NN - N98xNN (at least that is as high as I've seen so far) the odds are there will be no row 7 until they are done retrofitting. If there is a last second equipment change this of course won't help at t-24hrs.