They do not need to change the DTA. Example: beforehand no tax on capital gains in Thailand, now you have to pay capital gains tax on remittances. There is no DTA involved if you only live in Thailand in cap gains case.
Next example German pension: Beforehand TH has taxation right but did not tax it; after 01 Jan 24 you pay income tax on German pension if remitted to Thailand. And again in other cases, if you want to use a dta you must provide tons of translated documents and be willing to go to court. If the disputed tax is 50K USD and above maybe worth it, everything under 50K not worth the time and effort.
PS: May I ask you in which capacity you provide financial advise in order to make the above claim about DTAs in general? There are people on this board including me who work in banking and provide tax counsel in international tax matters FYI.