A software release was done in Jan 2010 that added DI to the SI units. Part of that softwere was some 2D enhancements. Those enhancements messed up the digital dept readings in shallow water. In June 2010, a software release was done to correct that. The non-SI units got part of that 2D enhacement which involved the Swicthfire feature. Your 728 has Switchfire. If you don't have the shallow water digital depth reading issue in less than 10', you probably don't need to update your unit.

I sounds like your 728 can be a GPS plotter with a GPS antenna hooked to it. If you wanted to, you can get another HDSI transducer to put on your trolling motor and switch units between the bow and console if you want to use SI on the trolling motor. The mount would be the only limiting factor which could be solved with a RAM mount for each unit with the same base size.

A software release was done in Jan 2010 that added DI to the SI units. Part of that softwere was some 2D enhancements. Those enhancements messed up the digital dept readings in shallow water. In June 2010, a software release was done to correct that. The non-SI units got part of that 2D enhacement which involved the Swicthfire feature. Your 728 has Switchfire. If you don't have the shallow water digital depth reading issue in less than 10', you probably don't need to update your unit.

I sounds like your 728 can be a GPS plotter with a GPS antenna hooked to it. If you wanted to, you can get another HDSI transducer to put on your trolling motor and switch units between the bow and console if you want to use SI on the trolling motor. The mount would be the only limiting factor which could be solved with a RAM mount for each unit with the same base size.

Unless I misread something, the interlink allows the 728 and the 898 to share a common antenna. 

Yes, if your 728 can be a plotter with waypoints, routes, and tracks, then a GPS antenna can be hooked to it either directly or thru Interlink.

You can get them for 4 bucks on ebay