Window replacement question

I have an aluminum clad casement window (actually a double window). I'm told that the kind I have new has a nailing flange that is nailed from the outside. The siding overlaps the nailing flange.
QUESTION: Lets say I wanted to replace the window. Would PROPER replacement of this type of window involve removing the siding and then removing the entire old window frame and installing the new window into the rough opening? Some home repair service told me that replacement could be done without removing the siding by cutting out part of the old frame and slipping the new window in. (not his exact wording) Is that really the BEST way to replace a window, or does the best way involve removing the siding and installing a new window into the rough opening ? My biggest concern is water intrusion since this is at the side of the building, and I've had that problem before.

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