The issue is families who want to sit together buying BE. But I don't see a solution of moving non-BE, non E+ pax up first. No reason to expect that won't create problems, including from pax who were happy to buy E, not pay for E+, and chose a seat they were satisfied with.
The only way they could really handle this sensibly is to take the labor intensive approach of asking pax in E- whether they'd like to move up to E+ in order to seat BE families together in E-.