I did the same, in the early days. 

I've upgraded both Windows 7 & 8.1, in its day, by using the downloaded Windows Upgrade Tool, I think it's called. I just ran that program, no USB USED. Apart from manually updating video drivers in 8.1,all went well. 

Everything worked as it should, no issues at all. I also turned UAC right down.. Too much nagging! The sims worked perfectly (I have 7 different FS2004) with the usual modified compatibility settings & tuning as Administrator. 

In fact, later today, I'll be getting my new laptop, and will be copying my stuff over from the old to the new laptop, using the using copy to a portable drive & then pasting to the new laptop. 

@Cfoley, I imagine you don’t visit AvSim that often if you haven’t heard of Prepar3D to give it its proper title. But don’t fear switching to it because it has the same structure as FSX with a few big improvements :-

Its now a 64-bit application not 32-bit as FSX is. In short, you’ll never run out of memory as you may well have done with FSX.

FSX needed a really fast CPU and a modest GPU. GPU controls the graphics.

FSX is 80% CPU; 20% GPU. P3D is getting close to 50/50 and it can take full advantage of a modern graphics card whereas FSX just can’t.

There is still a scenery library, BGL files and FSX aircraft on the whole will work in P3D. So will a lot of your scenery.

Most significantly, the performance in P3D will be a lot better than FSX can give you. Honestly, think about it. You’re stuck in the past with FSX and your new hardware will be wasted on it.