No need to change the clearance as there will be enough space for the 18" rim + tyre.
Have you already found a rim ? I would be suprised if you had found one with the same width as they are generally less wide than the originals.
Moreover, don't forget the amount of holes for it is hard to find a rim with 32 holes and not 36.
Apart from that, it is a good choice :
. offroading capacities will be increased
. a lot more tyres can be found for 18" rims
. & they are generally cheaper
Keep me informed !