I did this last winter, it went very smoothly.
I bought this house 5 years ago, and the existing Sears chain drive unit died. I took digital pics of the unit, and went to Sears. I found one that, other than the cosmetics, appeared to be identical.
Swapped it out with my father (in his 70's, but willing to hand me tools, and taller than my 5 and 7 year olds!) inside an hour.
Dead simple - take digital pics of the current wiring. Loosen the slack on the chain lots, and drop out the old unit. Bolt in the new one, and review how the old one was wired.
Nothing else had to be changed - I now have a very heavy box of spares (the rail, the chain, the sensors etc) in my cold room, just waiting for something to die next.
I would rate this a 3/10 for difficulty - it would honestly be a 1, if not for the ladder, and the need to hoist the opener up etc.