Can you change from synthetic oil to reg. oil without damage to engine?

Sure if you want to.
Best to completely flush your system to facilitate the change,

Many people will say that once you go to synthetic that you then can't go back to regular oil. I have done it and had no adverse effects. Just make sure you get as much out of the engine as you can and use a good quality oil that is specified for the car and it should be fine.

No problem. You can switch back and forth all you want--no additional steps needed. Toyota, now, only recommends full synthetic engine oil, due to major sludging problems with conventional oil in their engines. That's all they will give you, if you take it to Toyota. I recommend starting and sticking with fully synthetic lubricants--engine, transmission, transfer case, differential, power steering fluid, and grease.

Is your car still under warranty? Check with Toyota about how switching will affect a warranty. Switching in OK but you do not have to change as ogten with full synthetic oil.