Immigration will find it and not like it. If your motivation is to save the cost of a re-entry permit, they might see it as a way to beat the system and make you pay for it one way or another. They would not allow a person to be in Thailand under 2 different visas on 2 passports. They might even cancel your non-O.
sure. A visa on a new passport is as valid as a visa on an old one!
My point is when the IO sees that the old passport was still valid for 3 years, it might draw his attention. Changing passport to hide past visas is not exactly a secret trick...