Recently took the plunge and upgraded to Navionics+ using the bundled Gold card I got with the sounder.
Here's my experiences - hope it helps - any other input welcome.

1. You can use your bundled Gold card - though I was concerned there was some fine print somewhere. So for $175 retail you buy a Navionics+ upgrade card which you download the maps onto over the Interweb using the current Gold card as validation. You end up with 2 working cards.

2. The procedure - a little dodgy. Do not use the SD card slot in your PC even though the pictures show this. Use the USB adapter included with the upgrade card. When it says "Remove your card", it means remove the micro SD card from the adapter - not remove the whole shebang (like in the picture). Sort of counter intuitive to normal USB practice. This could be a local PC config thing but I tried it on both mine.

3. The result - well a little disappointing really. What I expected was basically what I get on my Android app - zoom in and you get finer contour lines from "Sonar Charts" and communitiy edits (where people upload hot fishing places that they want you to go and so leave alone their secret places.....) Zoom out and you get the normal Navionics detail.
Not so - at least for me.
With the Navionics+ card in the 7 HDI, you need to select "Navionics Fish 'N Chip" rather than "Navionics" in the chart selection to get the finer contour detail. Both charts are selectable. This detail only is visible on the lower zooms, which would be fine, but the higher zooms have almost no info on them. 26 foot contours and a few wreck markers. No community edits on any zoom level either.
So basically you need to swap back to "Navionics" to use the chart effectively when viewing a larger area and swap to "Navionics Fish 'N Chips" to zoom in. Strangely, you get the community edits in "Navionics" charts and not on "Navionics Fish 'N Chips"...

Lowrance firmware appears to treat the 2 chart levels as separate rather than an additional overlay the way the Android app does.

I'll seek some answers from Navico on these issues. Maybe I am doing something wrong.

Cheers