Sorta OT: Replacing windows & sliding doors in MD

I will be needing to replace 2 banks of sliders and am looking for info as to the best place to order sliding glass doors, french doors, and (eventually) replacement windows in the MD/DC area.
Each "set" of sliders consists of a 3' fixed panel with a set of sliders. There's a pair of such sets on 2 floors that desperately need replacing as they date back to the early 60s and don't exactly lock very well (at all in some cases so I will be relying on a pole in the track).
I've installed sliders in new construction, but any thoughts on additional issues with yanking out the existing stuff and installing new? I don't see it as an overwhelming challenge (remove door from frame; install frame; reinstall door into frame), but what am I forgetting?
Thanx Renata
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