At what point is it a new computer such that MS would be justified in saying I need to get a new license?
I seem to remember that there's actually a checklist of changes that MS uses to qualify a "new" computer. I'm sure someone can dig it up with some google-fu.
Something like; after 4 changes, it's "new". That could even mean 4 OS drive swaps though.
*maybe that was just a rumor and I'm totally off-base. I dunno.
TBH, first thing I'd do is just try entering your Win7 license when you install Win10 on the new machine. That's a pretty easy and harmless experiment.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod