Reuse Time Machine backup drive after migration to new machine
Apparently with macOS Sierra your new Mac can inherit your old Time Machine disk and likely with High Sierra, too. I recently moved the main drive from a failing 2011 mini to a new-to-me 2012 mini. I was prepared to erase the Time Machine disk and start over, as suggested by the first commenter, but I plugged it in and out of curiosity was looking at its contents in Finder. A dialog message popped up asking if I wanted this computer to inherit the Time Machine backup from my previous computer. It cautioned that if I wanted to do this I could no longer use the backup on the previous computer. Fine with me. My previous backup is now associated with the new computer with no problems.

Apple support doc outlining when and where you can inherit a Time Machine disk: https://support.apple.com/kb/PH25698?locale=en_US