During open enrollment, you can sign up for a marketplace plan even if you already have COBRA. You will have to drop your COBRA coverage effective by January 1, the date your new marketplace plan coverage begins. After open enrollment ends, however, if you voluntarily drop your COBRA coverage or stop paying premiums, you will not be eligible for a special enrollment opportunity and will have to wait until the next open enrollment period. Only exhaustion of your COBRA coverage triggers a special enrollment opportunity. (45 C.F.R. § 155.420).