The existing little card tickets are still valid (indeed I bought a new carnet of them not long ago on Eurostar); I last used a card ticket a few months ago. Some years back, I found a very old - unused - Paris ticket in my wallet, and it was too manky to work the gates, but the nearest ticket office happily swapped it for a new one. As far as I understand, you've paid for a journey, and they'll honour the ticket. The fact that they were cheaper when you bought them shouldn't matter.
By the way - although there's a direct metro connection GdNord-GMontparnasse (Line 4), my memory of it is that it's a massively long walk along several tunnels from the metro station to the main line trains at the Montparnasse end, especially since they expanded it to take some of the TGV routes. If you're not in a hurry, and want to see some sights en route, or have loads of luggage, remember there's a direct bus (the 91) which goes from a road (Rue de Saint-Quentin, I _think_) in front of GdNord [and can probably be be reached as quickly as the metro platforms], and finishes alongside the Montparnasse TGV station; it runs pretty frequently. I'm not sure how long the journey takes - it no doubt depends massively on the time of day. I've only ever used it between GdNord and GdLyon (which it goes past en route), and I think that took the best part of 30 minutes in busyish traffic - so the 91 route end-to-end might perhaps take an hour if you're unlucky with the traffic. However, if you have the time, it can (I find) be less stressful to use the bus than the metro. The same ticket is fine for either.