Take the receipt / letter from the labor office, go to immigration and cancel it. If this isn't done on the same day you can expect to get clipped for an overstay since your extension of stay was based on the canceled work permit.
If you want to switch from non-B (with WP) to SETV then you need you have your WP and visa canceled before you depart.
If they are not canceled then an exit without a re-entry permit will void the visa and you will enter on a 30-day stamp. You can extend that for another 30.
.
Go cancel your WP and then on your next hop you can get a SETV
OR
just out and in for 30 more then extend for another 30.
So... In for 30, extend for 30, repeat for 30+30 more. 4 months gets you pretty close to Sept.
OR
in for 30, cancel the old WP, get a SETV and use that for the next 90 (60+30-day extension).