There are less U21s games February onwards I think so it makes a lot of sense.
I expect Jackson and Kingdon to get a lot of U21s minutes from here, since I doubt we play Kwambala - Aljofree every game. We might finally see Mills, Armer, Kukonki on the U18s level more.
We also need to start pushing McAllister and Curley more to the U21s, considering we are left with only Collyer, Shoretire and, check notes, Huddlestone on midfield. Hansen's outgoing means Williams and Mather are probably first-choice for the U21s as long as Forson still with first team and Ennis still out, with Scanlon/Missin backup. And we still left with a 10 missing when Shoretire doesn't play there so I suspect Jack Fletcher will get some times too (ok we might just play Scanlon there).
So the U21s XI will probably have like half or more from the U18s when we add Nolan/Ogunneye, Harrison and Wheatley who are already starters there (Wheatley is second choice technically but McNeil is nowhere to be seen these days).
Kamason, Munro, Brennan, Ibragimov and Mantato needs to be U18s regulars moving forward.