It is likely that the height of your carpet and underpad is just shy of 3/4", so if you add the t&g plywood you are going to find your hardwood quite a bit higher than you will probably like. I replaced the deck of my subfloor so I had as level of transitions as possible. One thing you want to check is if the planks are glued down - the work load to replace the subfloor is huge. You have to weigh that against your preference for a level transition to the carpet. If you replace the subfloor I would put the t&g down as I went (don't strip the whole floor before starting to put down the plywood) I think this is better structurally. Hope this helps.