You should be aware of what your Merchant Marriner Credential says and know what it means. It makes a big difference to future upgrades.
If your route is “Inland” than you can upgrade to Near Coastal by completing partial exams, but if your route is “Inland limited to …” or “Limited to …” than you have a limited license and you will need to complete the full set of exams to upgrade.
Limited OUPV and Limited Master Licenses were issued prior to 2008 by completing limited exams. While these licenses are still valid to upgrade these licenses the holder must complete the full course or exams.
Contact me if you are unsure of what you have. I will be happy to describe it to you.