I've never encountered or heard of a GPU where you cant change the thermal paste. Some can be quite difficult like the founders RTX 2060 for example but it can be done on any card I am aware of.
Whether or not this will affect your warranty i don't know, I usually wont take apart a card until the warranty expires. I believe it is illegal for a company to void your warranty for simply changing the thermal paste, in US at least not sure about EU. Those warranty void if removed stickers you find on some GPUs MSI in particular are not enforceable in the US and likely the EU as well but I cant say for sure. Here is an article on it.
https://www.alphr.com/technology/1009030/warranty-void-if-removed-consumer-rights-nonsense