This has been an interesting series of announcements... I imagine this is intended to replace the oldest 37 or so MD-88s, even though a 192 seat A321 is a lot more capacity than a 149 seat -88. There will likely be some shuffling around with what equipment is used where come 2018...

753s take 763 routes (the 763s are getting retired)
199-seat 757s take 753 routes
739s take some of the M88/90/738/320 (149-160 seat) routes
738s do more transcon flying but still serve in the 149-160 market
320 does more of the routes that the -88s do (shorter haul)
319s stay in the 128 seat area
717s do more replacement of RJ flying, get introduced on more traditional CR9 routes, CR9s then take the -200/-700 flying.

I see the CS100 doing more current 717 flying, maybe up to what the 319 does now.

I think the 321 will be in between on various routes - could do transcon, shorter haul.. wherever it will fit in really. Similar to what the 757 is doing now.