You should take Tod Daniels advice very seriously as you could be on over stay after the work permit runs out. This could also affect further employment in the future if actions aren't taken quickly
You will need a letter from the Labor office before going to the immigration.( Proof of no outstanding tax)
If you are in Bangkok take your passport, contract of stay, visa and passport photo ( make photo copies ) download the form and complete ( then print out looks more professional)
Your Work permit and Non-B should run to the same valid date. The issue you will have is that late on when replying for a new Non- B there is no evidence of you canceling your work permit and visa. When the work permit expires so does the visa. I had an issue a few years back and left the country to obtain a new Non-B but I never told the immigration ( in which I didn't know the new rule) I got the Non- B outside the country but the problem arose when I went to extend the visa inside the Kingdom because of the work permit. Thankfully they gave me grace.