he might be referring to that other resistor on the side, I don't know if that was part of the vertical sweep mod or what, I got lucky I guess and all my U5000s had the upgrades installed already. but yes, the major one for changing between 15khz and 24khz is a big white plug in the middle of the chassis with a series of wires. it will actually be labeled on the two headers "15 khz" and "24 khz" denoting standard resolution and medium resolution respectively. if you have this, ONLY MAKE THIS CHANGE WITH THE POWER OFF.
if you don't have these, you have a U2000, which is standard resolution only. hunt around on the side, is there a label anywhere that says 25K50 or 25K20 or something like that? I remember they had some weird naming convention to them.