I purchased a Power Spec Desk Top Computer which had the choice of using a brand new Microsoft 10 or the Microsoft 7. I choose to use the 7. Now that Microsoft is abandoning all support for the 7 would I be able to use the 10 ?

You would have to purchase the upgrade to windows 10 and install it in order to use it.

That's info I already know. Since I choose the 7 before do I still have the option of switching to the 10 on the same computer without having to install a new software pkg

You should upgrade to windows 10 for continued support and upgrades from Microsoft. Microsoft support for Windows 7 will end 1/14/2020