How should I go about new computers pc

Parts for my computer are showing up between now and Christmas. The plan is to have it built before then because my old computer is supposed to be a gift to my sister. The m.2 4x4 wont arrive until 12/23-1/11 so I’m not sure if it will be here in time. I’ve also never ‘upgraded’ a system so I’m not sure how to move the OS over to a new drive/system so any info helps!

Would I take a noticeable performance decrease if I reuse my 3x4 nvme?

Should I wipe all my drives and get a new copy of windows or is it possible to transfer over?