ok,so the hotmaps premium i bought a year ago is now discontinued.
my updates are expired.
navionics is offering an update card for 99.00.
what exactly is the update card?? the navionics + ????
what is the main difference ?
if i do the update and use my hotmaps premium to activate it, do i now have two cards and the old one can no longer be updated??

I wound still use the old card. With maybe the exception of constantly rapid moving water, the bottom contour and locations of structure of lakes isn't going to change that much.

The Navionics Update card is Navionics+ at a discount. To use the data on the Update card, it has to be activated by another map card (doesn't have to be a Navionics card).
If you purchased a Navionics+ card directly, it is $199 MSRP. The Update card with the same data is $99. The coverage is the US and Canada.

During the registration/activation, your Premium card would be registered so it can't be used again to activate another Update card.
You loose nothing and end up with two functioning map cards.
The Premium card has never been updateable after the original year of freshest data updates after purchase expires.

A Hotmaps Platinum card is renewable annually and so are Navionics+ cards.

Navionics+ has Navionics maps (Navionics data) and has/can have the Sonar Charts (user generated data) map layer added. Hotmaps Platinum cards can have Sonar Charts added also.

The discontinued Premium product was ONLY Navionics data.

There is a new Navionics+ Region product that replaced the Premium Region cards at the same price--$149 MSRP.

Additional note about Navionics+ cards. They are customizable for the amount of content.

If you have a unit with a slow processor and all that data bogs it down, you can purge all the data off the card and only download the exact locations you need with both map layers.