Would WCG lose the benefit of any “Ready to report” tasks that happened to remain on my system as I reformatted the drive in preparation for upgrading the OS from Vista-64 to Win7-64? Would I lose the points such tasks had earned?

They weren't reported so yeah all points are lost.

No need to run empty before upgrading. If the data is already in C:\ProgramData\BOINC, just make a backup:

1. After hitting BOINC Manager, Project tab, Update button to clear any unreported tasks that were finished.
2. After stopping the client completely!!!!!!!!!!!!!!!1

Then do your W7 upgrade. Then prior to reinstalling BOINC pull the internet cable. Run the BOINC installer and point the installer to the old data_dir AFTER confirming that your data is still there. If not restore data first.

When you're satisfied that the client is running properly with the old data set, reconnect to the internet and on the go you will have retained your old device registration at WCG [has worked for me many times, but no guarantee]

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Every nanosecond... well, until last checkpoint saved of tasks running :D

Running empty on a multi-core can mean many hours of idle processors.