I don't think it's "just" an official change; whether a cause or consequence of the official change, I've encountered Czech people saying "Czechia" in English a lot more often in recent years than earlier. (E.g. the place I bought my bike from redesigned their website and the new one says "Proudly made in Czechia" where the old one said "Made in Czech Republic"). Of course these things are gradual but I do think it's a real change.