have a read up from the following link:
question is, what WP forks do you have or got, like a 43mm or 48mm and there are difference in the length of the diving tube..
well they are all marked to identify which one it is...
have a look on the top, near the tiny air bleeding screw... there is a number stamped in...
as a sample:
WP fork "1418W711" is a 48mm from a LC4 ADV 2002 and has springs with 4.4n/mm
a "0518W714" is a 43mm from a LC4 Enduro 2002 and has springs with 4.2n/mm
a "0518W722" is a 43mm from a LC4 SM 2002 and has springs with 5.0n/mm but a shorter diving tube and less travel length.
just to give you an idea, not to mention the size of the axle clamp and mounting brackets of the different brake callipers...
the easiest and cheapest is to change springs if you got the right fork legs in the first place to start a conversion.
so my advice... have a good look on what sort of forks you came along first, before starting converting it...
it's all depending on what sort of riding style you are after for... are you in need for a suspension for travelling, racing or SM-style... all factors matters.
have a look at the WP markings first, it can tell you the type and year of manufacture... post the number here and I can have a look what you got to guide you through.