I asked this exact question of United PR - he/she/they promised to get back to me on it in a week or two. I understand blocking the "old" E+ section (rows 35-44) in case of an equipment swap - but there's absolutely no reason to block the current E+ middle section (7 rows) when the old E+ middle section is the EXACT same size (i.e., in a swap, you could push those 28 seats back with no changes).

As a data point - my mother in law flew 896 last inght. Old E+ and new middle section E+ were blocked for months. Less than 24 hours out, they opened up both of those sections (though they went under airport control at 12 hours out). So once you're close to flight time, frequently check the seatmap.