I'd definitely do it and whoever it is either Millsap or Harris is fine by me - throw Bell in there as well if it's Harris.
Millsap
I really like him, but our future Trio has to be Jefferson, Favors and Kanter + 1 tall pesky back up Center. I hear everyone's argument about his character and his work ethic and how this would rub off the young players. But talent & size wins in this league and I don't think we can afford him any longer. Besides, the new guys we have (i.e., Hayward and Evans) already show a lot of character and work ethic which makes Millsap's intangibles a bit redundant IMO. You know Knight & Kanter are gonna hit the gym hard as well from pre-draft reports.
Harris
There's little doubt we should swap him for a younger version any chance we get. He is on the downward path in this career (losing his speed and leap) and he doesn't provide that veteran leadership that we need. For that why not just get Earl back for 2 more years to tutor Knight? I don't think he has a long future here IMO.
Are we going to be too young?
Yes and no. We can sign Earl (PG), we have Bell (SG - OK he's not the best, but he does provide the verteran cool head that we sometimes need), Miles (SF), Jefferson (PF/C).
Plus we'll have either Harris or Millsap as well (say we only trade 1).
So no, I think there's enough veteran leadership there.