Need another option --
Drop SG, bring PP
Drop SRT, bring RS

Kya karen bhaiya... we can't keep waiting on the corners all our life for that one inning to appear magically. It's time to take a stand. With great risk comes great success. I would rather put my money on two youngsters, than put it once again on old failures. If we had a 3rd replacement bat, I would have replaced RD too.
If the switch is based on current form, then RD has to be replaced by RS and not SRT. SRT has looked in much better form in the 2 tests than RD.
Well, RD and SRT have scored about the same number or runs. I think SRT has scored 5 runs more that RD. Now, in the middle order, we need someone who can stay. SRT has not shown any propensity in staying on the crease, while RD has definitely played more balls than SRT. So, if RD can stay on the crease longer than SRT and score JUST 5 less runs, I would go with him. If our line up for 3rd Test looks like VS, GG, VVS, RS, PP, RD, KKD..... it will look good to me at least. RS and PP will have the buffer of VVS and RD playing before and after them. There will be someone to guide them.