How would it not be hard? Those two cars are completely different. The 86-89 model's front end sits lower to the ground, the hood slopes downward, and it's got a boxy, straight-lined look to it. The 90-93 front end sits up higher, the hood barely slopes down (at least not as far as the older one), and has a more rounded and curvy look overall.
It's not going to be a "pull the old fenders, bumper, and hood off and slap the new ones on" kind of thing. You'd have to fabricate fenders using parts from both cars, you'd have to fabricate headlight and bumper mounting brackets, you'd probably have to fabricate the front cross-member. And if you can't do this stuff yourself (which I doubt you can since you clearly have no clue of how involved it would be), then it's not going to be cheap at all.
It's probably going to cost you at least a couple grand for a shop to do it, so I really don't see the point of it at all. You could spend a couple grand on this retarded conversion, or you could just spend maybe a grand or so and trade in your Accord on a newer one.
The choice here should be obvious...