Jake - You're 100% spot on. The Nav+ and Updates cards are the exact same thing except you'll need an older card to unlock the Navionics Updates card. Basically a trade-in program where you still get to keep your older card. Navionics view is that you've already bought into their product line and can continue your year subscription of daily updates and downloads for the same amount someone who just jumped aboard does which is $99 a year for the continued service.
When the card expires, you keep all the data you've downloaded. I fish all across the country and I only need one card to do so. Pretty sweet. Here's a few links to some videos I've done on the Navionics + card that may clarify and introduce you to a few more features.