On 4/13/2023 at 9:56 AM, CapraIbex said:

Or is it okay to wait (4 months) until 20 September '23

In practice, probably, yes. There is one issue, though. The 90-day reports ought to be based on your current valid passport (your new one). That will not be possible if the stamp transfer has not been done. Doing it online with the old passport number will usually work, but is not really correct. In your position, although a hassle, I would try to do my 90-day report in person, combining that with the stamp transfer.

removed a troll post.

On 4/13/2023 at 1:10 PM, daejung said:

What about if you got your new passport in your country and not at your embassy in Bangkok ? I your country they will not give you any letter.

The letter was supposed to be confirmation that you hadn't obtained your new passport from Khaosan Road, but a lot of IOs never bothered. When I did mine the IO never even looked at it, just threw it back.

If you enter the country on a new passport no confirmation would be required.