The '06 YZ250F fork are the same as the 450 fork except for spring rate and some valving differences. The 450 springs are probably too light for the 400 anyway unless you are pretty light yourself. The only difference is that the steering stem on the 250F is 12mm or so shorter. You should be able to press the stem down out of your clamps and press it into the 250F clamp to make it fit, but I'm not certain because of the age of your forks whether they share the same dimensions. Failing all else, Applied Racing and most other aftermarket triple clamp suppliers will sell you the stem as a separate part. All KYB twin chamber forks come with plastic free pistons, not just the '05's. If yours are currently OK, there's a way to prevent them from breaking.