If you get a new 899, as mentioned above that will have both side and down imaging. My understanding is that you can attach an external GPS puck to the 899, even though it has an internal receiver–this makes your way points on the Stern transducer more accurate, so its a good thing. I bet your existing puck plugs right in but hummingbird customer service can verify. Also, your 997 has the high definition transducer, which is better quality than the one that comes with the 899–as long as the transducer itself is in good shape I would leave it in place or youd be downgrading by reinstalling the new one from an 899. On the new models from hummingbird the high definition transducer starts with the 900 series (although if you have an unopened transducer from an 899 they will switch it out for a high definition one for a fee, I think $75) If your units are connected by an ethernet cable, I believe you also need an adapter for the new 800 series, possibly even for the new 900 series.