KTM400-
NO, removing the base valve will NOT allow you to separate the inner and outer tubes to replace the seal.
Remove the dust wiper from it's seat. Under that, there is a circlip - remove the circlip from it's groove and take the back-up washer with it.
Now, with the oil drained (unless you want a big mess) and the fork cap and spring removed, you "slide-hammer" the inner and outer tubes apart. That is, grab the outer with one hand and the inner with your other hand and yank the two in opposite directions. The seal and the bushings will come out with the inner tube.
You don't necessarily HAVE to remove the cartridge. I'd also be careful removing the base valve with an air wrench, if you decide to do so. I think KTM sent out a bulletin about base valve removal on these forks.
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