high speed differential nets crossing on PCB
When it is only a single diff-pair, the typical solution is as follows, no layer changes required:



With many diff-pairs or large busses of signals, it may not be possible to keep all the tracks on a single layer. In these cases, you should do your best to minimize the number of layer changes (and hence vias), and control via impedances as best as possible. With a well designed ground plane and nearby companion vias, it is possible to design vias that preserve the trace impedance.