The 757-200 and 757-300 AVOD/slimline projects aren't exactly new topics.
Two and a half years ago for the 753:
http://www.flyertalk.com/forum/delta...ng-thread.html
More than a year for the 757s:
http://www.flyertalk.com/forum/delta...-aircraft.html
As for using mainline or bigger RJs to replace CRJ-200s, DL is fully five years into that effort.
The OP might not be able to keep up but the
AJC certainly should.