When opening a specification from an older version of the software, you are not asked to validate.
By default the NBS software is NOT set to automatically check for validation - this is to ensure the user is not prompted to validate every specification they open if they don't wish to.
The option to check for validation may be turned off. In order to turn this feature on, go to Tools > My Settings > Keeping up to date tab. You will then see four tick boxes to use for the different validation options.
If you wish to turn validation off, simply un-tick all 4 check boxes and click OK.