Can I change my TR visa to a Non-O visa without leaving Thailand?

It can be done inside of Thailand, but you need to check with the immigration office and ask for their list of requirements. Some require the required money to be in the Thai bank account for 2 months already at the time of application, but some only require to the money to have been deposited and no amount of time. But at all offices you must have at least 14-21 days left on the existing stamp in order to apply, or else you'll need to get an extension or leave and return to get more time.