After first attempting to upgrade my project from 2018.1.3.6784 to 2019.1.2.7018, the whole import process hung for hours.
Then, I deleted pretty much all but one of my .unity scene files, and the migration did eventually succeed.
However, ALL occurrences of the wwise scripts that were attached to my objects now show that the script is missing.
This is about the 4th time I've attempted this upgrade. I've had to debug the process a significant amount to figure out why it's not working.