Bob,
If you go to Navionics site, you'll see that they now have a different upgrade process where you can buy just the upgrade, not the totally new CF card with charts. This link should help you:
http://www.navionics.com/en/navionics-updates
I did this about 1-2 yrs ago and it was an easy process. If I recall, the upgrade is for your existing charts. For example, I have all east coast and the upgrade applied to all regions on my card.
In regards to Bahamas, even though navionics east coast charts cover it, my advice is to get familiar with GARMIN BLUE CHART MOBILE. Since you've mentioned that yours is RM plotter, then your best bet (like I do) is using iPad and load the Garmin software. GARMIN has the best charts for Bahamas. I've used them both for my trip to Abacos and GARMIN was definitely the primary and most detailed source. It was nice having Navionics charts on my Dual E120s for confirmation and comparison.
What I also liked a lot was the suggested routes (magenta line) on GARMIN. Navionics has them as well, but very limited compare to Garmin.
