I suggest that you read all of this.
A word of advice:
You should always have your data backed up to at least two external media destinations (external hard drive, DVDs, flash drive) in case of system file corruption and/or hard drive failure.
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And you should always do this, not just before upgrading, etc.
These are quite good programs to use for Complete PC Backup and are free:
"EaseUS Todo Backup Free"
http://www.todo-backup.com/products/home/free-backup-software.htm
"Welcome to EaseUS Todo Backup Manual"
http://www.todo-backup.com/support/help/
http://www.todo-backup.com/download/docs/User\_Guide.pdf
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"AOMEI Backupper Standard"
http://www.backup-utility.com/free-backup-software.html
"Configure and automate AOMEI Backupper in Windows"
https://www.winhelp.us/configure-aomei-backupper-in-windows.html
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"Macrium Reflect Free"
http://www.macrium.com/reflectfree.aspx
"Macrium Reflect Tutorial"
http://www.macrium.com/webtutorial/tutorial.asp
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You do not say what type of 7 Pro licenses they are.
If they are OEM System Builders licenses, they are only for a clean install, not upgrades.
You do not buy a Product Key by itself, as they are normally stolen/pirated, Volume Licenses which are not for private use, or MSDN keys which are not to be sold by the original subscription holder.
Make sure that you buy the complete sealed package (DVD, license)
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About sales for 7:
https://support.microsoft.com/en-au/help/13853/windows-lifecycle-fact-sheet
How long can retailers sell a prior version of Windows?
We will continue to allow retailers to sell the previous version of Windows for a year after the launch date of the new version.
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Download Windows 7 Disc Images (ISO Files) - Microsoft
Microsoft doesn't supply them for OEM preinstalled licenses (Dell, HP, etc.).
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If preinstalled:
Or you can use this for preinstalled OEM, System Builders, or Retail Licenses. This is on a first come, first served monthly basis.
Download the correct ISO file and make your own disk and use the Product Key that is on the COA Sticker.
Download: Windows ISO Downloader.exe
Cheers.