I'm going to buy a yoshimere or WB pipe for my 400EX. Will I need to rejet or not. I've heard of people having promblems when rejetting when it gets cold and that it's not worth it, I live in PA. Any suggestions would be appreciated!

2000 400EX soon to be modified

 

As a general rule most engines require a fatter main jet when increasing the air flow in any way such as a pipe, airbox alteration, etc. Many pipe manufactures suggest using a main jet two sizes fatter than stock, then testing for performance at 3/4 to full throttle to detect any skipping, immediately after which pulling the plug for a read as to color. If the insulator or electrode appears dark or black, the next leaner jet can be used & retested. The danger is not in running too rich, but running too lean, thereby increasing heat & the possibility of major engine damage in severe cases. Main jets are cheap, $4 or $5. Pistons, rings, & cylinder re-borings are not.
BTW, in extreme cold weather you could very well have to re-jet anyway, with or without a pipe. It's not that difficult to do.