You can eventually become ineligible for IBR by your income increasing to a point where you would be at or above your standard repayment (at which time your payments become your standard repayment). Whether this can happen by prepaying some on your loan is a little less clear to me, but presumably if your debt burden is so low that your income to debt ratio is high enough that your payment would equal or exceed the standard repayment you would then be placed back on standard plan. See the thread where someone had their payments increase by paying 4K on a 40K loan. I guess this is what happened to them (especially if they count that extra money as income-that OP got the money from his parents). You payments would still count towards PSLF or the longer loan forgiveness, but the trick is that you would likely pay off the entire loan (or much of it) before forgiveness time happens (since the standard repayment is meant to pay off your loans in 10 yrs)