You can downgrade at any time before the 25th of any month (which is when billing is generated).
Maybe consider though that the standard rate for 1GB Mobile Data is R99. So you if you are on Mobile One, the extra R99 pays for your double data, and then the extra 500MB and simfy plus 2 for 1 topups are Free. If you're on 2GB, then you're getting two GB's for R99 (R49.90 ea) plus everything else free.
So it's actually pretty easy on the pocket if you think about it that way. Also, you're getting double data (on Mobile Two and higher) at a per GB rate that is the same or cheaper than top up rates on 2 for 1, so it's definitely worth weighing up before you make your decision :)