Substitution reaction and configuration

[Thecoolone]

Homework Statement

(2S,3S)-2-chloro-3-methylpentane + high concentration of CH3O- ---> ?

The Attempt at a Solution

I need to find the product and usually my method is to knock off the -Cl replacing it by
-OCH3 and when drawing the compound I just make an mirror image. In (2S,3S)-2-chloro-3-methylpentane there are two asymmetric centers and for the product with Cl replaced with OCH3 I assume that it changes configuration so that it becomes 2R instead of 2S. My problem is now, that I cannot draw the product without getting (2R,3R).
Is this correct? I assume that it should be 2R,3S because I do not knock off any atom or molecule from the second asymmetric center.

 

[chemisttree]

It is a secondary halide adjacent a tertiary carbon center. You sure you're going to get Sn2 product? How about E2?