I am doing the WJ Knuckle Swap on my 98 XJ and the more I research, the more confused I am getting about which unit bearing needs to be used for this conversion. Looking to see if someone can enlighten my on this.

I am seeing that the 99.5+ unit bearing needs to be used for this swap due to changes in the rotor casting for those model years. Not sure I understand why, but see it in a lot of threads. Why can't I use the unit bearings I already have for my 98? The reason I ask is because they are practically brand new and I hate to buy something else that isn't necessary for this swap. Is this just to avoid having to use the 1/2" spacers on the caliper bracket? What's the downside to using my original unit bearings?