Tod is right, if you are going to other country which has visa on arrival they will check your return destination, if you return to Bangkok they ask you got visa or work there, so carry it
Depends on Passport and company type. If its non BOI company and you hold Passport from countries where the visa need to be changed in their country only (Like China, India, Bangladesh etc..) you can't Change here. If the company is BOI they can change visa to Non B irrespective to passport you hold
James is right, get letter from doctor that you are not allowed to travel for next xx number of days, happened this to my friend before, and there will be immigration doctor also to check, you can show the stitches.