From a technical standpoint, raising the backflow preventer looks easy. There appears to be a threaded nipple between the two ball valves on the inlet side; just replace that with a longer nipple.
However, if you want to avoid that for aesthetic reasons, be aware that burying the flow meter will hamper RF transmission. If the controller is more than e.g. 50 feet away, I suggest testing for reliable transmission before committing to buried installation. Also, you need to provide sufficient clearance for replacing the batteries and viewing the status light.
Yet another option is routing the output of the backflow preventer up and back down. This is also somewhat ugly, though the top of the pipe will be ~2" lower than the black cap would be with the backflow preventer raised.