the three young starters will work additional innings about equal to one additional starter. Added to Tork and Greene that will be the equivalent of three new players. Add 3 FA distributed at SP, SS, C and it makes a turnover of 6 players out of the core of 14 (5 starters plus 9 hitters). That enough turn over to create a lot of possible improvement (or disappointment!). Alexander probably also adds to the rotation but I'd guess he'll be a wash performance wise with Boyd or Peralta or whoever you want to consider him as replacing.
And that doesn't include the BP, which is a bigger piece of the puzzle every year. But like any BP from year to year it may go either way. Possible downsides are Cisnero and Funk turn back into the walk machines they have been in the past; another Fulmer injury. Possible upsides: Lange, Roni Garcia, Faedo, Wentz, Madden, Jimenez finds some command, etc....