Basic window retro fit is easy, even in a brick veneer. I do it all the time. But, let me ask you this, what product and material are you looking at;....wood, clad wood, vinyl, aluminum?? Also, be very leery of the window departments at home depot and lowes. You basically give them the measurements, they order, and if you are wrong, then you bought windows that either dont fit, dont meet code, etc. Stick with a professional that does windows everyday, will meet you onsite, give architectural suggestions, and hold your hand throughout the process. I do this as my vocation, and would be glad to give you my input.