Mozart's woodwings serenade , Key Change

[bilgekaana]

Hello everyone, I have a question about mozart's woodwings serenade.
In the first movement , bar 31 . Mozart changed the F note to F sharp.
But the key change is E flat major . I didn't quite understand which key did he pass to ?
I looked in the circle of fifths and other things, but I didn't quite understand why did he use F sharp.

[quantum]

In a question like this, it would be beneficial to include more specifics on what the actual piece is.

K.361/370a for reference.

Bar 29, cadence in a tonal centre of B-flat.  Bar 31-37 sequence leading to C (cadential 64), ending on Bar 39.  Bar 40 arrive at new tonal centre F, the dominant of B-flat. 

Bar 30 chord is G minor, Bar 31 chord is D7.  D major is the V of G minor and F# is the leading note to G minor.  This is all part of the sequence. 

[bilgekaana]

Thank you!!