There are no software updates after softwre 0650 for the pre 2010 navigation units. So I think basically any disk from 2008 onwards has no updates to software for RNS-E, only map updates.
If you have a unit with X--- for the software version then you probably have a hacked version for SDS, go into the sound menu and look for the speech dialogue menu, if you have it, then it's probably a hacked unit.
If you want SDS then really you don't have any worries, but if you want to get rid of the X--- and the SDS menu, then downgrade the software any other lower version by using VCDS to enable a software upgrade/downgrade and then after it had downgraded insert your new disk and it will upgrade to SW0650 and the X--- will be gone along with the SDS hack.
If you want to get rid of the X--- and keep the SDS menu, then downgrade the software any other lower version by using VCDS to enable a software upgrade/downgrade and then after it had downgraded insert your new disk and it will upgrade to SW0650 and the X--- will be gone along with the SDS hack. Then use a proper hacked disk for SDS with 0660 software index (downloadable from tinternet), this will update to SW0660 and install the SDS hack and the version will show 0660 and not X---.