The headlamp units are perfectly interchangeable, only problem you may encounter is that the old ones wont come out. The bolt are fixed in the plastic frame, and when the nuts are rusted, the power required to unbolt them can be high enough to loosen the fixings...
Ask me how I know and how a quick headlamp swap turned into a small nightmare and ended with a broken lamp.
Another thing to remember when buying upgraded headlights, is that North American versions do not seem to have the parking lights in the assembly. I know when I changed mine several years ago I purchased a new set for a good price from the USA on eBay...it wasn't until I went to put them in that I realised there was no parker lamps, and the boss/moulding where the parking lamp usually goes was closed over. I decided that I would live without parking lights, and put them in anyway...but after a month of "Parking lamp Failure" messages every time you turn on the headlights I got out the dremel and opened up the back of the holes and installed the lamps (luckily the bayonet moulding is still there under the closed off hole), and also added the extra pin in the connector from the original set.
As has been said they swap straight over - having said that I upgraded my fog lights recently on my 1998 (everything else had been done) and the fit into the body is a bit sloppy - alot of daylight coming through - I can't recall if it was any better on the old ones but it looks to me as if there should be some sort of seal around the recess or simply a better fit - a bit sloppy I reckon.