I'm going to quit my job on Friday and return to Canada on June 1st. I assume that my employer will take back my work permit. As I understand it, once I quit, I have 7 days to leave the country. Is it possible to change my non-imm B visa to a tourist visa without leaving Thailand or will I have to do a border run and return so I can spend my remaining 3 weeks in Thailand on a tourist visa?

If you are on a 90 day visa entry you can use the remainder of that permitted to stay. If you have been on an extension of stay from immigration it ends at end of employment and you must leave that day from recent reports (no more 7 days) but you can obtain a 7 day extension of stay from immigration with proof of employment termination. As you need three weeks would suggest you make a border visa exempt trip to obtain a 30 day stay if you are not currently on a 90 day visa entry that will cover your time.

Thanks. Looks like a border run then. At least it gets me out of Lopburi.

I can confirm that as of today at Suan Plu if you are on extension of stay your visa ends on the day you finish work. You can get a 7 day extension with a letter from your former employer showing the separation date, and it better be the same date as you turn up at immigration.