On a younger (well maintained ) engine there's not really an issue going from full synthetic back to regular.

The issue is on older engines that have always run synthetic. The additives in the synthetic oils help keep seals intact and prevent degradation. So on an older engine if you change back to economy grade dino oil that does not have these additives, you're likely to generate leaks at a quicker pace. There are however some non-synthetic oils that are meant for high mileage vehicles that contain the additives as well.

If in doubt, an oil analysis is a great way to determine the "health" of your engine.