You can use the HD500 to select different Variax settings on the older models, but any alternate tunings will need to be built into the Variax setting.
There is an input option for Guitar + Variax which will do what you want.
I use both my JTV69 and Variax 700 with the POD HD500. The HD500 can't change the tuning for the Variax 700 but if you only use the Variax with the POD, you can customise all the User presets in the Variax with whatever guitars and alternate tunings you want because with the POD you can also access the Factory default guitars (that come with standard tuning).
Another difference is not being able to mix modelled guitars with the native magnetic pickup tones. The JTVs are a dual channel guitar and you can do cool things like mix an acoustic (modelled guitar) with an electric (magnetic pickups) guitar.
So, I'm sure you'll get good use out of an earlier Variax for the time being and that might give you an appetite for the extra flexibility that comes with a JTV.
If you do get get a JTV later, keep the old Variax as a backup.