Yes, as long as you do so during the Open Enrollment period.
If you are enrolled in employer-provided retiree health coverage, and if your income is at least 100% of the Federal Poverty Level, you may also qualify for premium tax credits. However, there’s one exception. Some employers may provide retired employees with access to an account, called a health reimbursement arrangement (or HRA) that the retiree may use to reimburse medical expenses, including an individual policy through a Marketplace or in the non-group market. A retiree that signs up for an HRA offered by a former employer is considered to have minimum essential coverage from an employer and would therefore would not be eligible to claim a premium tax credit if he or she enrolled in a Marketplace plan.
Remember that outside of Open Enrollment, you cannot voluntarily drop your retiree coverage and replace it with other Marketplace coverage.