reading this thread it's a little confusing, so to clarify:
you cannot "upgrade" multiple lower value credits for a higher value credit...so, for example:
If you have only snack credit left, that's all you can get with them: snacks. You cannot combine snack credits to get a counter service or table service meal. Now what some people do is make light meals out of snack credits, like for instance getting a breakfast by using 3 snack credits to get : coffee (1 snack credit), muffin (1 snack credit) and banana (1 snack credit)
counter service credits cannot be combined to get add up to a table service meal. No exceptions have every been reported.
Sometimes (emphasis on sometimes...not always, and not everywhere) but sometimes you may be allowed to "downgrade" leftover credits. Like you may be allowed to use a table service credit at a counter service place. YOu will only get one counter service meal for one table service credit so it's not a good value for your credit, but it's better than leaving the credit unused. By the same token, you may be allowed to get 2 or 3 snack items for one counter service credit...but not always, and not everywhere. People report the best luck doing the downgrades at the resort counter service. Some have reported being allowed to do that at some park restaurants, too. So always ask before ordering to make sure you won't get stiffed at the cash register.
Clear as mud, right
