For this reason Adobe don't automatically change all your old images to the new process as you may well be perfectly happy with them as they are. Given that you can't undo the changes once made you might want to consider creating a virtual copy and updating that to the new process.
For a Lightroom user, it is in any case possible to go back in the History which will show a step "update to current process". Clicking before that in the sequence, will return the image to the Develop settings that were then active, applying those settings under that Process Version once more. You can switch between process versions as you wish, it is all reversible.
Whatever settings then do not appear, are still remembered in the background - so if you have a particular "fill light" setting in PV2010, and then click the exlamation mark icon to go to PV2012, you may need to use other sliders to achieve a similar result as the now missing "fill light" parameter. But if you then go back to the PV2010, your fill light will reappear on the same setting as it had before (though the image may have changed due to ialtering other sliders meanwhile).
The same should apply to ACR, except you do not have the same History function available there. However if you make a Snapshot before updating to the new Process Version, this snapshot will record (and therefore be able to later recall) the earlier PV used, as well as all the settings that go with that.
You can use Snapshots in LR too, if you prefer to preserve the state just before updating PV explicitly, with a name.