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Hi Jim,

After migrating to new Mac, you have to reinstall AutoCAD (uninstall it and install again) - this is true in all cases.
But you have 2015 version which is not supported on Mojave, so unfortunately you will not be able to install it. The earliest AutoCAD for Mac version compatible with Mojave is 2017 with 2017.3 update installed. If you cannot downgrade macOS, the only one way is to upgrade AutoCAD to latest 2020 version (or at least to 2017).
Plus keep in mind that 2017 version is not supported on Catalina, that is why I recommend you upgrade to 2020 version.