You can definitely change the actuator with the turbo in place but I remember having to remove the manifold and the water feed banjo bolt to gain access to one of the bolts for the actuator bracket.
That was when I had a k03s, I didn't end up using the forge actuator for the k300, the k300 was mapped with a 1 bar actuator that came with the turbo, also not sure the forge actuator from the k03 would for the k300 turbo because the k300 uses a k04 compressor housing, not a k03 housing.