The valving in the SX will be set up for landing big jumps; it won't be as good at soaking up slippery tree roots as the EXC. Lotsa suspension shops do a roaring trade making MX suspension more supple for trail use. If they are the same series fork (4860MXMA?) then you can probably drop the SX springs and pre-load adjustable fork caps into the EXC forks. You might have to check the length of the cartridge and rebound adjuster tube if the fork caps are different heights below the fork tube tops. If they are different, then you might also have to swap the EXC rebound (mid?) valve into the SX cartridge. Assuming the fork springs are the same length...

I was able to use 2005 640 valves & cartridge for the damping and travel length, 2003 200EXC fork tubes for the 200's axle & wheel, and 2004 950S springs that matched my weight, all together on the 640. Three cheers for modular builds!