I am not a John Deere owner so I cant answer if it is in-fact identical. I know In Versatile tractors the 335, 375 and 400 all run the same block and are supposedly just tuned different. I have always assumed that as they increases power in each model they put better components (especially seals and rings) but as I said ASSUMED. Non the less this doesn't change the fact that it affects your life time. If the 8200 and 8400 are in-fact the same block I would assume that the 8200's simply outlast the 8400's becasue there is less stress on them. BUT if the blocks are the same it means the tractor can of course stand the stress, this without doubt has been tested extensivly through JD, I would just expect a shorter lifetime. As a note you should know I have a bias towards running things slower. Our farm almost always buys a size to large and runs it below full capacity, things just last longer when they turn slower; tractors, tires, augers, pumps....
Out of curiosity why are you looking for more HP? larger implement?
We are the same way in that we normally buy larger than what we need and take our time. It is our only solid tractor on our farm so we want to keep it in good shape. Only has 3300 hours on it.
We are looking to disc rip a little with it since we picked up a farm which allows us to do so in the Fall. Would be nice to bump the horsepower up to 225 / 240 to handle a 5 shank Deere 510. We will probably keep it for a very long time and start using it as a planter tractor, etc whenever we can afford to get another tractor with more horses.