I suppose your edition doesn't specify a tempo change there? Czerny's edition marks the 3/4 section as "Allegro", which might not originate from Bach himself.
It's of course finally about understand what sort of composition this is. What is obvious is that in a sense we have here sort of three movements in one: the opening, marked Grave Adagio, then the 4/4 Andante, and finally the 3/4 section.
Since Bach entitled the movement "Sinfonie", he is thinking about a particular sort of composition. Without doing more research, I can only guess that he is thinking about a form of orchestral piece, say an "Overture" for orchestra, maybe in an italian style, which might be structured just like that: slow introduction, then an andante, then ending the piece with a faster movement. But everything built into the same movement, so there's no "paus" between the movements.
You should do some research about what is behind the title "Sinfonie", and I'm sure you'll find some answers there. Also, Bach might not have cared to be more specific because in his own days it would have been obvious what sort of "style" he wrote it in...
Instinctively, I'd say you should speed up things in the 3/4 section, based on the notion that it is actually three movements in one, and the music itself seems to require a faster tempo compared to the Andante that precedes it.
Topic: Bach partita no. 2 - help me please