while you may be able to use the new Lowrance units on your current blue-cap cable systems, one of the big advantages of the new units by both brands is the ability to connect them with ethernet cables instead.
these ethernet connections allow you to share much more real-time data between the unit on your console and the one on the bow...including the sonar displays from the other unit, or the gps data from the other unit's antenna, etc... (ie., view the SI image from the back of your boat on your bow unit, view the 2d sonar reading from your bowmount on your back/console display, or view the gps location of the bow of your boat on the display in the back...think fishing/guiding from the back of the boat but knowing precisely what's going on under your trolling motor up front.)
if you're upgrading both bow and console units and want to take advantage of the ethernet advantages, you'll need to install a new network backbone with either brand.
in the case of the Humminbirds, these ethernet cables will/should be compatible with a Terrova iPilot in the near future. imagine being able to plot a route around a piece of structure on your gps map and then tell your trolling motor to drive that course all on it's own.
of course, if you have every spot on the lake memorized and you just need a small sonar to watch out for the rocks that moved over the winter, none of this probably matters anyway... ;-)