"Otherwise I was feeling OK with going to the station and getting it modified two days out."
That's great, but then your initial reservation ought to have been a regular permit, no TC exit. Having that wrong TC exit gives you no advantage for getting the right one as a walk-up, since that decision will be based only on whether there are available walk-ups and meanwhile your reserved exit will have gone to waste. It *might* give you an advantage the other way around, if you tried to get a walk-up for Bishop Pass 4 days earlier than your entry. In that case they might honor your TC exit date and allow it to be transferred to the walk-up Bishop entry, but I'm not sure if that's what they would actually do.
Not faulting you. I'm just saying that everyone loses with the current system since this is what nearly everyone does, I suspect.