I think it goes without saying that teams should try to address their biggest needs first, and while there are some exceptions that I readily admit, in general you should try to fix the biggest problems. Looking at last season I think it's easy to see the biggest problems, here's who they were by PFF's ratings
Brooking/Bradie -12.0 (Bradie -4.1 Brooking - 7.9)
Terence Newman -9.1 (after a solid start, no less)
Phil Costa -9.7
Doug Free -11.9
Abe Elam -5.3
Given that Free was the single worst player on the team by this metric last season, I have some reservations about moving him to RT and thinking that will solve the problem. He'll still get his doors blown off by JPP, Cole, Babin, Orapko, and the like. Perhaps the strong side will over him some extra help, but keeping a TE to block also comes with its own set of problems. It looks like three of these players have been effectively replaced
Dan Connor had a +4.9 rating and assuming Carter is average (which I think is safe) that's about a 17 point swing at ILB alone. Pool's about average and there's another five points. Carr rated as as +2.7 and there's another net gain of 12 points. So total they've added about +34 in PFF ratings just on the defense. That's a huge improvement, I think Carr can represent a bigger improvement given his other advanced stats: 9th in passer rating allowed, 31st in targets (despite playing opposite an elite corner in Flowers), 22nd in yards allowed, only three TDs allowed, and 18th in yards per cover snap, and oh yeah he's still improving. Over all I'd say the defense has gotten the makeover that a lot of people here think that it needs. The OL is still a work in progress, a lot will depend on Free's performance at RT, what they do in the draft, how the kids and the new FA signings perform, and what they do at center. Overall if it's the defense you're worried about, I'd say your worries should be quashed, however if it's the offense that you're concerned about? Well lets just say there's some nice first round linemen projected to be there around 14.