I am starting a new job in May and normally I had to go to Laos to get a new non-B in April. But it looks like this will be impossible as borders close (won't be able to get into Laos) and getting back into Thailand will be equally difficult.

So I am looking for alternatives.

I am still on a non-B from the previous job and have a work permit which probably will get cancelled during April as well (but my previous employer has no intention in rushing towards a cancelation, they understand the situation I am in and are supportive)

I have read before that it should be possible to get a new extension from my new school without leaving the country by cancelling the WP, cancelling the current extension and applying for a new extension on the same day. But I have never actually met anyone who managed to do this.

I don't know if it is true that this is only possible if you stay in the same province (and deal with the same IO). In my case, I would move to a neighbouring province.

 

If anyone has any experience with this or advice whatsoever, it would be greatly appreciated.

It is possible to change employers without leaving the country. It requires cooperation from you current and new employer.

Your new school will need to apply for a work permit to get a work permit application approval letter and you will need a teachers license or a waiver.  They will also have to supply all the other supporting documents. Then with a letter from your current school stating the last day you will be working for them you could apply for new extension on or before the date on the letter with the letter and etc from the new school after your current extension is  canceled. After the extension granted your current work permit could be canceled and your new one issued. 

 

Just wanted to give you a quick update of what happened yesterday. So I went to Chonburi immigration to get confirmation if transferring my non-B would be possible and what the procedure would be and which documents they require. First of all this was the absolute opposite of social distancing, and it made me worried afterwards : if I ever got infected, I'd know where I got it from. After queuing up what seemed like forever, I was directed from one Immigration officer to the next. The last one finally gave me the answer I  kinda expected (although I'm not sure he even heard or understood my question, but just gave me his standard answer) : NO. 

But he kindly reminded me that if my current non-B gets cancelled, I'll be paying a 500-baht overstay fine per day.

 

It was a frustrating experience, but it was worth trying. And I got the outcome I anticipated. The worst thing to see though, was how other foreigners who got bigger and more urgent problems were treated. I could use a whole list of adjectives to describe that, but I guess you can imagine that for yourself.

 

Anyway, seems like my only option here will be to wait till borders open again and then apply for a new non-B. Don't know when, don't know when

You should not accept what one immigration officer told you.

It is certainly possible for it to be done as I wrote before. You would not be the first one that has done it.