If your subs is no longer current all it means is you can't get updates.So yes you can keep using what you paid for so long as it's already INSTALLED.As for OEM numbers object desktop is a suite therefore separate serials isn't required unless you buy them as stand alone is my understanding.
You may be able to reinstall a product when subs have run out but only up to the point you originally had a running sub so subs run out in feb 2015 you MAY be entitled to updates that were issued in feb 2015 but not after,however because it's subs based it would be up to stardocks discretion.Obviously for actual facts about what's can & can't be done it'd be best to contact stardock directly as what i've written here is what i've seen in the forums over the years
edit: $30 per year works out at 57c per week,let the sub lax for a year while saving you'll have the money in a years time,however there are the odd sales & member sub give away's if a member has too many
