With your later addition of Roberto, your assessment of returnees to be counted on are spot on, but besides another large freshman class, I don't anticipate a "major roster change"--at least not one involving multiple deletions. At forward, six returnees--Hohmann, Rodrigues, O’Regan, Lane, Baillargeon, Roberto--plus three freshman--Eichel, Phelps and Greer (who is accelerating)--should comprise the top nine. Another freshman forward is likely as well. Of the other six, Duane, Collier, Moran, Kelley, Lawrence and Ryan (better suited at forward and there's a logjam on D), the first three could form an effective fourth line. Even with a fourth freshman, that's 16 forwards and especially after this season Quinn isn't going with fewer than that. So, we should expect a talent upgrade with the three (or four) highly-regarded freshman arriving, while both departing seniors never played above the fourth line.
Four Ds are coming in with MacLeod and Hickey likely the most ready for prime time, while Diffley and Abramson are late bloomers who have excelled this season. Grzelcyk and Okasanen will be regulars and probably Somerby, too, as you suggest. MacAfee is an enigma, who played effectively early on--showing good judgement on pinching and a knack for getting the puck on the net; now inconsistent and not playing as confidently. But will Quinn start 3 freshman on defense? That's risky even with talented kids. Regardless of who gets ice time, that's eight defensemen--a full complement for a change.
24 skaters and 3 or 4 goalies (if Moccia returns). If one or two players are not invited to return, it's getting pretty late in the year to add multiple players who could be contributors. Unlike this season, competition for ice time should be extreme. The roster change will be nine freshmen arriving, providing more talent and depth.