Theoretically it does not matter as long as you provide proper coupling between different reference planes. The following animations illustrates the current flow in transmission line, Wikipedia gif. Moving dots illustrate local charges. As one can see, the charges oscillate along conductors, including the reference plane. When you switch planes, you need to shunt the gap with appropriate capacitor, to provide continuity. The coupling with MLCC caps might be tricky however, since the impedance of caps can turn into inductive branch for high-frequency signals, so the caps need to be selected carefully (and placed in near proximity to microstrip trace).
This animation also illustrates why any discontinuity in ground plane are bad for characteristic impedance uniformity - cuts in reference plane are obstructing the current oscillations.
ADDITION: Here I found a nice article about the fundamental importance of Return Current" in HS design, which partially touches the topic of split reference planes, "The Ground Myth", see Slide 93.