I think it would be very hard or impossible to answer your question in the abstract. You really need a chimney expert on site to look over your particular set-up and make recommendations. Having said that, I will contradict myself in part by suggesting that you pose your question to the forum in the website named HEARTH.COM. A fireplace aficionado on that forum might give you an opinion on what is likely to be realistic in your case. But in my limited experience in alterations of different kinds, even in relatively young houses, when you start to take something apart to plan to do this or that, you often find surprises, usually not good ones. That's when you find out that what you want to do will cost a lot more than you planned.