You can just send an e-mail and they switch your account over in a month. I can't answer the question definitively about whether you can make the 30 days notice part of the 6 months timeline... But the way it reads:
"...an Exclusive Contributor may terminate their status as an Exclusive Contributor with thirty (30) days prior written notice to Dreamstime.com. If an Exclusive Contributor terminates this Agreement prior to the passage of six (6) months, Dreamstime.com reserves the right to..."
I would say if you are giving 30 days notice to terminate, then you haven't actually terminated prior to the 30 days being up... I would want to clarify with their support, though, as you stand to lose all the exclusive earnings and upload bonuses if you get it wrong.