Awyrennwr, adverse-bump
There is no need to talk with ATC in Class G when changing between IFR and VFR, nor is there a need to receive any service at all. While you may find that ATC offer to upgrade your service from FIS to RIS, or offer RAS if you say you are IFR - there is no requirement to avail oneself of their services.
The phrase VFR on Top should be used with care because it means different things to different people. It could be flying VFR above a cloud layer or in the US it could also be flying a specific type of IFR clearance above a cloud layer remaining VMC and complying with the VFR semicircular altitudes (with the altitude selected at pilot discretion)