A:Answer Yes, yes and yes.
I have used 2019 MS Office to open and modify both Word and Excel files created using the 2003 version of MS Office that I have on a DVD. It is a perpetual software license.
I was suspicious about buying the download version; but, no one seems to have DVDs of the new versions of MS Office these days. The literature I have read says that it is possible to reinstall this software on another computer by contacting Microsoft and giving them the product code for your software. They will deactivate it; so, it can be installed on another computer. The product code is the 20+ character code you use to install the software on your computer. I strongly suggest that you write the product code in more than one place; so, you can find it when you need to.