We have gone the "phased approach" over the last two years. When we bought our boat it was pretty much the original electronics kit from 1996. Everything worked... but I was really used to a much higher level of functionality and user friendliness that it could supply.
Our initial change was to go with a B&G Zeus Touch 12 MFD mounted in a Navpod at the helm. The NMEA0183 output from the Hydra was connected to the MFD input. The MFD takes that, and echoes the data out on the NMEA2000 bus for use by anybody else who needs it. That meant that the new and the old played together well, and none of the old instruments ever had to be replaced because of communication incompatibilities with the new.
The vintage Furuno GPS became redundant, and was removed.
We added a B&G Rate Compass, and a B&G AC42 autopilot computer.
We eventually added 2 Triton displays. We removed the original B&G Analog Depth display, and kept everything else--for now. They were really great displays in their day, but I hardly ever look at them--the Tritons supply way more information in a very usable format.
We added a B&G Sounder module, and a new transducer to get us better information about what's under the boat. We take our fishing pretty seriously, so that was important!
A CS4500 speed transducer was interfaced to the Sounder Module, and has proven to be a functional and reliable device. Our sonic speed still works, after a fashion. But it very susceptible to electrical interference from other devices so we don't rely on it much.
When the Furuno Radar died, we dropped a B&G 3G radar in to replace it, also a change we are very happy with. Having the radar display at the helm suits the way we handle the boat much better than having it only at the nav station.
The original wind instrument works, and we will keep it for the foreseeable future. The Hydra's only real function at this point is to accept and transmit that wind data to the MFD. I haven't seen a wind instrument that adds enough functionality to the system to justify replacement of all that hardware and wiring.
Once we had the NMEA2000 backbone created, adding things incrementally has been pretty easy. The boat came with a Raymarine AIS650, that was only ever connected to a laptop for display. Connecting its "Seatalk II" output to the NMEA2000 bus was just a matter of a simple conversion cable, and it was working with everything else happily.
At this point, we do not have any significant electronic wish list items. The newer generation of Triton displays did not add enough to make us covet them. The newer MFDs also haven't jumped far enough ahead for us to be unhappy with what we have.
Better sonar is always something I am looking for...
I think the key take-away for you is that none of the changes we made forced the timing to make others. They were pretty much all independent decisions, and everything worked together as the changes proceeded. The key was getting the Hydra data re-broadcast on the NMEA2K bus. That made everything else easy.
LMC, Fort Lauderdale, FL