When Kepler was an assistant of Tycho Brahe, did Kepler in public declare his support for Copernicus' system or Brahe's system (the earth at rest, but the planets orbiting the sun) or was he undecided? The latter is told in the novel by Max Brod: "Tycho Brahe's path to God". But that's a novel only.
Kepler never supported Brahe's system in print. In his first book (1596) he proposes his own version of heliocentric system. Collaboration with Brahe is 1600-1612. During this period he wrote Astronomia Nova (1609) based on heliocentric system. And all his later books are based on heliocentric system as well.