To keep my phone number linked to a DTAC Standard SIM and migrate it to a Nano size SIM for my New Nexus 5X ,  I just go like down to Siam Paragon DTAC store?

 

I'm assuming I won't lose my phone number etc ? 

 

 

Yes they can fix it no problem - if they have to they can clone the old sim to a new one and provide a nino or just punch out the current sim - they may want to clone it to ensure you have a sim that is 4g able even if your phone is not yet.  Free.

Yes, it's free and quick. They simply re-map your exiting number/account to the new SIM, with a few keystrokes and the scan of the bar-code on the new SIM. 

 

 

Some phone shops have a very low-tech solution, a plastic cutting device, like a hole punch, that trims the sim down to the smaller sizzes.

Take your Passport with you :)