Do think you are right, but when I moved from Dtac -> AIS, I did not have to go or talk to Dtac, AIS did everything for me and I received SMS from both providers on my number to keep me informed.
They (both) also notified me in advance that on that day and on that approx time I could swap the SIMS. Down Time was kept to a minimum.
I also received Sms from Dtac for the following
- a sms with a link so I could cancel the whole process
- a sms with a promotion that If I cancel I would receive some Gb for internet for free.
overall a smooth process.
(however YMMV)