"If I install from CD for tax year 2018 which I have saved, it won't update..why would I need it to?"
Because the program on the CD is a preliminary version. It might not have all the forms you need, the calculations might be incomplete or inaccurate. It needs to be updated to work properly.
"But it will still open all files and worksheets related to that return? Or it won't work at all?"
It might not open all the forms and worksheets because your tax return (the .tax2018 file) was produced by the updated software, but in your scenario you don't have the updates anymore.
"If left on PC from the original time it was installed, it will work?"
Yes, for 2018, provided that is was updated after the original installation.
