Gas to diesel, i/o to outboard, two-stroke to four-stroke (or vice-versa), etc.
10 years ago my inlaws repowered their 31' Uniflite from Chrysler 440s to Mercruiser 350s (non-efi). Today we have 900 hours on the meters and the engines still look new. Performance didn't suffer and the boat sits a few inches higher in the stern. The bill was $25k. Fuel economy remains around 1 mile per gallon no matter what we are doing. She tops out at 31 knots, we cruise around 20 knots.
Currently I am in the middle or repowering my 23' Uniflite. I have removed the old OMC i/o and sent it to the dump. I may loose faith and take the remaining hull and trailer to the dump as well.