I think it's NOT that the rest of NCT Dream members would promote under NCT U after this comeback. It's just the system of NCT Dream would be flexible like NCT U but only with the 7 members of the original Dream members.

So, in the future NCT Dream may comeback with 7 members or 6 members or 5 members etc. It can be anyone who's absent from the said comebacks, but they're not joining NCT U (except Mark and Haechan who's already members of NCT U)

As Mark and Haechan are also members of NCT 127, Renjun, Jeno, Jaemin, Chenle and Jisung can also join other NCT units made in the future. So they're not only members of NCT Dream unit.

And, the graduation system was cancelled. The rotation system will only include these 7 members. It's OT7 forever yuhuuu