Yes and no.
Expansion cards: Yes, all expansion cards go through a Balancing Phase where they are adjusted to ensure they aren't over- or under-powered. No, expansion cards won’t be changed after the Balancing Phase is complete.
Free to Play cards: Core cards and Welcome Set cards, which are considered “open” sets, will always continue to be balanced on an ongoing basis as these are cornerstone when it comes to keeping the game balanced as a whole.