Although I don't have the Legend I have the Vista. The same message came up when I loaded Map Source Topo and Metro Guide.
I didn't lose any info that I could find in both the city and in the woods.It may be just a default letting you know it may overwrite ?
I have the same GPS as you. I get the same message when I download software upgrades. Don't worry about it! It is safe to download to your GPS. And if worse comes to worse you can download the program from Garmin to restore the original operating system.
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