Please switch this into another forum if inappropriate.

I understood that the purchase and new registration process of a Thai (AIS) pre-paid SIM card guaranteed its "ownership" indefinitely, even if the credit ran out and top-up is made after the active period has passed.

Is that correct or have I got it wrong?

Moved to the IT, communications and etc forum.

I think all that SIM card registration guarantees is that the "government" can tie a mobile number to a person, only for "security" reasons of course !

If you let it expire they will probably sooner or later put it back out for sale.

I don't know about Thailand but in the UK, a pre-paid SIM card only remained with the user as long as it was periodically used. I had an elderly relative who had a mobile phone with pre-paid sim for emergency use with all relatives programmed on speed dials. The phone had to be used at least once every rolling 13 weeks (either one incoming or one outgoing call). Failure to use the phone in this period meant the phone number could be re-used by the phone company and any credit remaining was lost. I used to phone the mobile every month just to keep it activated.

Once the active date is past, you no longer retain the number.

All pre-paid SIMs have an expiration delay after your credit expires.

Every time you top it up, the expiration date of your credit is pushed back.

A certain time after this credit has expired, the SIM will be invalidated and the phone number recycled if not topped up.

It's my understanding that this certain time = 6 months for AIS.

Of course this does not mean that the SIM will be "put our for sale". SIMs have a unique identification number that is not reused. The SIM will become invalid because it will be dissociated from the phone number. And this phone number will be allocated to a new SIM.

I don't know about Thailand but in the UK, a pre-paid SIM card only remained with the user as long as it was periodically used. I had an elderly relative who had a mobile phone with pre-paid sim for emergency use with all relatives programmed on speed dials. The phone had to be used at least once every rolling 13 weeks (either one incoming or one outgoing call). Failure to use the phone in this period meant the phone number could be re-used by the phone company and any credit remaining was lost. I used to phone the mobile every month just to keep it activated.

That information will come in handy for the OP.

Any idea what happens in other non-related countries?

After the "valid until" date the number is deactivated, registration has no bearing on it.