Hello there,
You can open the card up to reapply thermal Paste; This will
not void the warranty on the card.
There is a sticker on the back of the card over one of the screws, this is so that we know if the card was opened if it is sent in for an RMA.
As a side note, there are a few things that can void our warranties, few examples are, removed serial or manufacturers stickers, liquid damage or physical damage.
More information here:
https://www.evga.com/warranty/graphics-cards/If you do plan to replace the cooler with a water block or a third party cooler, you will need to keep the original cooler.
In the event that the unit needs to be sent in for warranty, the unit would have to be in factory condition with all components.
Hope this helps,
Robert - EVGA