In my experience, for trips on hold, while they may reprice the whole trip, you can retain your held fare buckets on the legs you aren't changing. So if you have SFO-DFW-GRU on hold in S, you can change the held return without losing that on the outbound. If the price increase is due to SFO-DFW-GRU no longer being available in S, you'll be ok. If the price increase is due to a change in the published fare for SFO-DFW-GRU in S, you'll have to pay the difference.