F-2 to F-1 while Spouse's I-140 pending

[mushal]

I am on F-1 STEM OPT status where as my wife is on F-2 visa which expired last year. Both her and mine I-20's are valid till Aug 2018. My H-1B application has been picked up in lottery this year and change of status is in pending status. I haven't filed change of status to H-4 for my wife yet. My wife has got admission in a university for her masters. Now, we want to apply for change of status for my wife from F-2 to F-1 so that she can study and work as a teaching assistant. As my NIW petition (I-140) is pending and I have included her in my I-140, will this be a problem for her to change of status from F-2 to F-1 here in the US and/or applying for F-1 visa from my home country? Will this change of status from F-2 to F-1 while I-140 is pending be considered as visa fraud? I'd appreciate your advice. Thank you.