Opus 12 isn't really 'heavier' than Opus 9 so I doubt you would want to do that, unless the laptop is running a really ancient version of Windows (Win95, Win2k) which newer versions of Opus no longer run on.
But to answer the question, quoting some parts of the Licencing FAQ:
[ul][li]Your Opus 12 licence can be used with Opus 11 (or even Opus 10 or 9) if you really want.
[/li]
[li]Please email sales@gpsoft.com.au if you have a question relating to licencing, purchasing or upgrading that isn't covered [by the rest of FAQ].[/li][/ul]