My PC never updated to build 1607 - Had a valid digital license.

davexnet said:

My system prior to W 10 was Vista and two installs of XP. About a year ago, I got my data out of Vista and downloaded
the Windows Insider build, formatted the Vista partition and installed clean using a key for insiders at the time.

The system updated with new builds over time, eventually reaching the official release version when I dropped out of
the program. According to the Activation status it was activated with a "digital license". The activation persisted,
up to build 1511. . At no time did W 10 prompt me for a qualifying W7 or W8 key.

It wasn't until I had the motherboard swap that things went wrong

The 64 bit W 10 Pro is about $129 at Walmart - have not checked Neweggs prices yet, but it's similar I'm sure
If i'm going to do it, may as well do a fresh install.

Yep, there was a point in time early on, when W10 was first released to the general public, that Insiders were auto-activated, even if they had not officially upgraded with a qualifying key. It was a small window of time, and you evidently were in it.

Yes, a fresh install is always good.

davexnet said:

Then I have to ask, if my activation was valid due to being an insider at the right time and place, would it have lasted
indefinitely assuming no hardware changes?
If yes, perhaps I had a case when I called MS? Seems as if they treated it as an oem Vs a retail licence regarding
tansfer to a new motherboard. Is that what the digital license is? Like oem ?

Yep. It was a small window of opportunity, - a loophole - and basically a lucky opportunity for a small few. Changing MB and CPU constitutes a new device. That's why you lost the activation. It's no argument to use with MS; it won't work.