I looked into it on my old boat, and in addition to the new switch/gauge, it also required an updated unit (sending unit) at each tab. This was with Bennett hydraulic trim tabs. I declined, and just replaced the switch. My express has the indicators, and it is a *nice to have*, but I'd still not add them if the switch went, I just don't get that much utility out of them.
It's called EIC. It involves removing the rams and installing a sensor rod in each ram. This is the installation instructions. It's an easy DIY project depending on the ease of access to the inside of the transom on your particular model. Really a good upgrade.
I don't know about the hydraulic Bennetts . Years ago i had a bot with Lenco electrics and I fitted the LED controls . All i did was unplug the none LED and plugged the new LED controls . The plugs were the same .
On another note I prefer electric tabs as they are much quicker to respond, the hydraulics are very slow .
I've been looking to do this on my 265 and if I could have figured out how to access the hydraulic lines to add the sensor rods to the rams I'd have done it (or tried to do it) already. However, in past conversations with my local GW dealer I've learned that the rams on my boat are custom from Bennett and none of the regularly available online kits would work for my application. It may be hogwash but I doubt it and given the challenge of actually getting to them and the quote I got from the dealer to do it for a bit over $800 I'm going that direction this off-season.