Get your PSSR rep out from the local Cat house and have him do a full undercarriage survey. It doesn't cost you anything and he can tell you within a few minutes if your rollers have enough life left in them to support a new set of rails. With the tools those guys have these days, they can measure to within a fraction of the actual metal remaining.
You gotta remember that matching old parts with new always means that one side or the other or both are going to experience rapid wear initially untill they get seated in with each other. This reduces the life expectancy of the new iron some.
If for some reason you are not buying Cat parts, the folks you are buying from should also have someone that can measure your U/C and give you a report on where it's at wear-wise.
I wouldn't put the new stuff on till you have a report in hand and the rep's opinions.
Beyond that, trying to get the last fraction of an inch of wear off one individual part isn't always the best policy cost-wise either. It can wear the new parts unevenly and accelerate their wear. It can break suddenly, costing you lost production where you have to send trucks and crews home. You know it will never break in a good place, but always in the middle of a rain or snow storm in 12 inches of mud. It will, no matter what, mean that you have to take the machine out of production sooner than other-wise might be the case to change out those rollers as they start leaking or break.
About the only time I ever left used rollers on was where the rails were being turned and the wear was such that all the pieces were going to be done at about the same time after the turn.
But, like I say, get a survey done and then you will know exactly where you stand.
Good Luck!
