They will start to working on the 747's starting summer of 2011 through summer of 2012. There are 16 of kind, so its about 1.3 planes per month.
JFK may get lucky to get the first retrofitted plane this summer, but positioning/scheduling may inconsistent. (NRT-JFK-TLV-JFK-NRT)
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Below is detailed time line provided by delta blog
+ 767-400ER flat bed : by mid 2011 (21 aircraft)
+ 777-200ER flat bed: by mid 2011 (8 aircraft)
+ 767-300ER (76T) flat bed: by mid 2011 (7 aircraft)
+ 747-400 flat bed: by mid 2012 (16 aircraft)
+ 767-300ER (76L) flat bed: by mid 2013 (45 aircraft)