No, you do not HAVE to remove the damper/cartridge assembly, and, yes, the Yamaha Service Manual will show all of the steps to completely disassemble the fork.
The thing is, you do have to be careful not to allow the damper rod to extend out of the lower fork tube while handling it and inadvertently whacking it against something and bending it, scratching it, or in some other way molesting it.
Just search YouTube for "WR250X fork seal", and you will see videos of what you want to do.