If you change both the primary and secondary you will have a hard time reaching any local network resources, so if you have other networked computers, network printers, network storage, etc. you'll probably want to leave your secondary DNS as your default, if you don't have many/any local network resources changing both is ok.
Overal it shouldn't hurt anything so long as it's a reputable company. They could technically direct you anywhere on the internet with fake DNS and get you fake banking sites and stuff and steal your log in info.
But if a google search proves them trustworthy you should be fine.
Just remember to change it back to your defaults, or to google DNS when you are done with the world cup. You don't want your DNS queries to be coming from half way around the world, it could add a lot of latency to your internet traffic.
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