I received my HDS-12 on Friday, and got it installed over the weekend.
A pretty straight-forward transfer! The power cord and transducer from the HDS-10 are compatible with the HDS-12.The gimbal bracket for the HDS-12 is only about 1/2" wider than the bracket for the HDS-10, and the bolt hole spacing is the same, so no knew holes required. My concern about having room for the bigger bracket, was valid on my console, but I had room for the new one with 1/16th of an inch to spare!
The biggest challenge of the transition was the Structure Scan. The LSS-1 unit for the HDS-10 was a separate unit, but Structure Scan is built into the HDS-12. It turns out that all you need to do is unplug the Structure Scan transducer from the LSS-1 unit and plug it directly into the HDS-12. This required enlarging the hole in the dash for the cables, because the connector for the transducer is a larger diameter than the Ethernet connector.
The only disappointment is I lost my 400+ way points. I should have done a back-up of the way points to an SD chip BEFORE my HDS-10 died!
Rick K