Well, I can still hear the quotes from both departments officers in my head after that whole odissey: „Take care not to let any of those documents expire as they will cancel each other if one of them expires“. This is actually what happened to me. The only thing I am saying is to better fix everything NOW before expiry and don’t try to fix things later on. It will most likely get expensive and tedious.
well, maybe things in Phuket are different then. Actually this little detail forced me to renew all my documents on the expensive way at that time. WP ran out, therefore non B got cancelled before expiry and I had to re-enter Thailand as a regular Tourist. Then take all my documents to Penang, apply for a new non-B, fly back to Phuket, make a new WP, spend thousands of bahts, loose time and miss other jobs also. Remember that there are two players: Immigration and also the Labour Department. They have different rules but can both affect both documents.
Remember that when your work permit expires then your visa will expire also on the same day (and vice versa). I had exactly the same job/visa situation 3 years ago. You need to apply for visa/wp before you leave and run out of time. Please keep in mind that your passport might be taken in to bkk immigration. As i live in Phuket, I will always need to refrain from traveling for about 10-14 days due to that.