I have a 98 400F and the lugs on the bottoms are leaking. It is alot cheaper to by another set it seems so I was wondering what other years are able to be swapped out? I know 426 years but what about any 450 years? Thanks.

All YZ/YZF '98-'03 will slide directly into your triple clamps, and are fundamentally identical (except for the obvious springs and valving) to yours.  '04 YZ/YZF are are a larger, 48mm version of the same fork, but will bolt onto your frame by using the '04 clamps.  '05's were the first twin chamber YZ/YZF fork, and can also be used with their matching clamps.  '06-'07 (except YZ250F) can be used with their own triple clamps.  '08 and later (and '07 YZ250F) can also be used, but you'll have to adapt the brake caliper to the fork or use the complete front brake from that year model.

The front wheel should be usable in any of these with the original spacers.  In some later models, you'll need an axle to match the fork because the "fat end" is smaller.

Thanks you for your extensive knowledge once again. I have one other question. If I was wrong about the lower lugs leaking what would cause oil to get by the new seals in the upper section. Would i possibly need to purchase new upper tubes? Sorry I 'm very nonversed in the all might forks. Thanks for your time.

Seals may leak for a number of reasons.  Loose fork bushings allow too much movement between the upper and lower tubes for the seals to "cover".  Dents, nicks, scratches deep enough to be felt with a fingernail can cause trouble as well.