I disagree with the point above. There is probably no EASIER projection for a coach to make than the one moving from OT to OG. If you thought Light was a good run blocker, he'd be the same good run blocker as a G. If you thought he was excellent in pulling on bubble screens, etc, then he'd be just as good pulling from the G position. If you thought he was a good pass blocker with a weakness with certain speed rushers, then he'd be even BETTER as an OG, who rarely has to deal with that issue.
There is a VERY good chance that Light would be an EXCELLENT OG and he'd be paid a very handsome wage to be one for the Pats. Certainly enough that what he might have made elsewhere for a year or two, would be covered by what ending his career as a Patriot would do for his outside income going forward.
There are a lot of risks going forward for a 33 year old guy like Light even if he could get a better contract INITIALLY with another team. That team would ALWAYS be looking to get younger, different systems and coaches could be detrimental to someone who spent his entire career with one coach, and one system. It might not be an exact apple to apples example, but look at what happened to McNabb when he left the Eagle's nest. Its not easy going from one system to another. Staying with the Pats, even at a discount, might be the best long term move for Light.
That being said, this scenario is NOT likely, especially if the Pats decide to keep Mankins. However if Mankins goes, THEN I think it becomes a LOCK.