"Do you think its too late to go synthetic at 144k miles?"
No, I don't think so. Our 1MZ-FE (Sienna) had 262,190 miles when I finally made the switch from conventional to MaxLife full synthetic 5W-30. We're at 276,300 now. I've found that the choice of oil filter makes a huge difference, too. After experiencing a lot of valve clatter after the first start each morning, I learned that only a filter with a silicone anti-drainback valve will do. Wix and Purolator PureOne are my preference, and I hate the filters in orange cans!.
"do you guys believe in once you go synthetic you can't go back??"
Not really. I wouldn't be afraid to switch back but I'd keep an eye on things to see if anything changes.