F1 change to F2

Hi, friends,

My OPT will expire on Sept.I plan to change my F-1 to F-2 shortly before the expiration.
My hu*****and is gona submit I-140 and I-485 concurrently very soon(end of this July). My question:
Should I change F1 to F-2 and get the receipt before I-140 & I-485 submitted. Or it is ok to change to F-2 after I-140 & I-485 is pending?

Thanks a lot!