I thought it was going to be standard from this year - ie sometime in 2000 the 757-300 interior would be applied to the 757-200 as standard.
If you think about it the 757 lent it's cabin design to the 737-3/4/500 when they were launched - saving money for Boeing - all it's narrowbodies have same interior.
When the 737NG was launched, and later the 757-300 it was logical they should share the same cabin design. It makes sense to apply this to the 757-200 now they have stopped making the 737-3/4/500 - again all narrowbodies will have the same interior.
Also, I read that at the same time the 757-200 switches to the new interior, the 767-300 will switch to the new 767-400 interior design - again for the reasons above - plus, while the 757-200 cabin design still looks very modern and "curvy" the 767's interior has dated and really needs bringing into line with the 777/764
I'm open for corrections on the above - I don't recal the exact facts