Do we need to re-register our TM30 in Samui after traveling to Bangkok?

Ask Samui if you need to reregister.

Usually you don’t. Some offices don’t even care if you leave the country and come back (though that’s not happening anytime soon)

In THEORY (at least the way that the amnesty revisions were written) IF you are going back to your primary residence (meaning the address where you have a TM30 filed) you do not need to re-file a new TM30 for travel inside the country OR coming in from outside the country.

BUT with that being said

Some immigration offices are still requiring you DO file a TM30 for inside the country travel SO sadly the answer to your question is at the immigration office you use. I would call OR go ask them.

OR

Post in a f/b group related to Samui, :)

If a hotel in BKK re-registered you, you need to make a new one again to get your old address back. If not, not.