There doesn't seem to be any such sinister plan going into place yet....the yet-to-be-delivered 739s are coming with 20/42/117 configurations, which are a step down from the 20/51/96 originally configured when CO metal had E+ added, but still have 20F. The blocking of rows 3, 4, 5 is simply to allow flexibility for anything from a 319 to a 739 to run a flight months from now with minimal changes in seat assignments. It's for the same general reason as for renumbering the coach rows so exits are 20 and 21--so that people don't find themselves losing exit row seats when there is a swap (sometimes as late as at the gate, which blocking rows 3-5 months ahead is not going to help with on the day of departure).

Looking at the narrowbody variants flying today, if you're booking E+ and want it to stick, avoid rows 9, 12, 14, and 15. They have the potential to disappear completely, revert to E-, or who knows what. Same principle as booking row 1 or 2 of F if you feel a swap could still be coming so your row doesn't disappear.