Question looking at getting a wr250r

I started to look at finding a wr250r and have run across an x. Can I just change the front to a 21" wheel and leave the 17" rim on the rear? I assume I would have to slide the front forks down to compensate for the height. Found a dicent deal on the x so wondering if this is possible or would end up buying a back rim also. I see the front brake is larger on the x but not too woried about that.

thanks for any info

I think some folks just put knobbies on the X and dual sport it.

thanks for the reply