Coming back to LOS with a many year DTAC number and wanting 3G if possible, I wonder if I can port my number across to another network these days.

If so, it is free, painless and how long does it take ?

Yes you can transfer your number from one network to another for free, all you have to do is go to the AIS with your passport and the documents that you gave DTAC. if you can't remember what id documet you gave DTAC, then take your passport and ask them to update your record they should be able to print it out for you (that is what I had to do when I switched from AIS to True), then go over to AIS and give them your passport and any other papers then wait about 3 - 5 business days for the transfer take place. You can still use your mobile while the transfer takes place. I would suggest going to Siam Paragon to do it as there is an AIS and DTAC store there, so you wouln't have to go far between the two.

Of course, AIS is not the only option to which to port your DTAC number.

MNP'ing to True is pretty easy, as well.

You might experience a "dead period" once DTAC switches off your service until your new provider switches on. I was out of service for about four hours when changing from DTAC to True.