I did that upgrade last season for the same reason on my 240. The good news is the i40 panel mounting hole is the same size as the ST30. I really like the larger display size. I purchased a used i40 off ebay that came with a newer speed/temp transducer. So I replaced the old ST30 speed transducer when I changed out the display. I tried using the old depth transducer with the i40 for a season- they are supposed to be compatible, and worked fine most of the time, but the depth would occasionally freeze up and read 1.8 ft. for extended periods. Then randomly go back to reading correctly. I checked connections but never could figure out the problem. I called Raymarine and they suggested replacing the old depth transducer, so this spring I bought an Airmar P7 shorty depth transducer, installed that, and it has worked great.
So you could try an i40 display with your old transducers and see how it works. Note the 1998-vintage ST30 transducers on my 240 required 1.75" holes, and the newer transducers all need 2" holes. So I had to enlarge the holes using a hole saw mandrel that can mount 2 hole saws nested, a 1.75" to center in the old hole, and a 2" to cut the new hole. Worked great- bedded them with 4200 and no leaks!
