Upgrade? Are the new ones not HD? I was unaware there were more than one type?
The Helix 5 and 7 SI models have 83/200 kHz 2D sonar and 455 kHz Imaging. So they come with the Compact SI transducer XNT 9 SI 180 T.
The Helix 9 comes with the Compact SI transducer and is factory set for 83/200/455 kHz. BUT it does have the 455/800 kHz Imaging capabilty that the HDSI transducer is tuned to. Cheaper out of the box with the less expensive Compact SI transducer.
The Helix 10 and 12 SI units come with the HDSI transducer XHS 9 HDSI 180 T and are factory set to use 83/200/455/800 kHz.
ANY Side Imaging unit regardless of their Imaging frequency capability that comes with the Compact SI transducer, will get sharper images using the HDSI transducer. Just adds more cost.
The reason the images produced are sharper is the HDSI transducer is 2" longer and has longer SI crystals than the Compact SI transducer. Those longer crystals produce a thinner SI pulse, and the thinner SI pulses produce a sharper image. The sharper images are more "HD" than the less sharp Compact SI images. Plus units with 800 kHz using the HDSI will produce even sharper images but with less coverage.