You have to maintain F1 status throughout even if you get TPS with EAD. For that you have to maintain minimum credits (with TPS, you do not need to be full time student). If you do not maintain F1 and do not change to any other status before the end of TPS, you will be out of status. With 18 months of TPS and chances of extension, you have to change to other status (COS) like H1B, Green Card etc.