The switchover is needed when they do the cutover on forecast.weather.gov, then the old V4.02 script will stop working, and only the V5.nn script will work. They've set June 19th, 2017 as the next cutover date. DaculaWeather posted that message from the NWS
here.
Outstanding issues with using the advforecast2.php V5.nn are still:
1) no Outlook type and only some Statement type alerts appear in the api.weather.gov JSON feed
2) still some issues with Watch/Warning/Advisory alerts too.. sometimes duplicates, sometimes just missing.
3) may continue to be foibles with the text forecast (like rain amounts when snow is forecast) with the JSON point-forecast from api.weather.gov.
I'm hoping that all those get worked out in the remaining time until the June 19, 2017 date...
I'm going to leave advforecast2.php V5.01 on my site (and in the template distribution) for now. The updated DualImage.php (and icons) are compatible with both V4.x and V5.x of the advforecast2.php scripts.
Do make sure your URLs all have lat=...&lon=... values in them to be usable with both old and new scripts, otherwise it will fail with the new (which only uses the lat/lon values as they are deprecating the MapClick.php script for the new forecast.weather.gov site)