Ok....let me be the first noob to ask this...
I have had the notification from Microsoft, and reserved my upgrade. When this happens, how does it work. What about all my e-mails, documents, and especially stuff that relies on registry entries. Will it just overwrite certain bits, and carry on as normal or will I have to load programmes back in.
Please let the answers soothe my troubled brow. 
Oh...and I have 3 computers running 7, will I get notification for all three ?
Will be a totally fresh Install that will overwrite your existing OS and keep (Try to) your existing programs , probably with a Roll Back feature to your existing OS (there be silly not to put that in).
Personally, assuming you have enough Disk Space kicking around, I would backup / clone your existing OS Drive(s) and run with a Dual Boot for a while until you feel comfortable with it. Then do a completely fresh Install of 10 (using your existing Win10 Licence, if needed) on a new drive and reinstall your programs and data.
And yes, Win10 will be available for all your OS's running 7,8 and 8.1 that are capable of running Win10, Hardware dependent, so will exclude Win7 Mobile, Win8 RTM etc.