Yes you can do it, and no you won't have to reinstall the rom. A hard reset will bring you back to exact moment right after you finished flashing the custom rom. You will lose all your text messages, call history, and any changes you have made since flashing the custom rom. I advise you look into backup/restore solutions if you want to save anything, the most popular of which is PIMBackup:
Dotfred's PimBackup