This may help:
IK PM User Manual wrote:7.3.2 – “Reinstall Library” button
Click on the “Reinstall Library” button to start the download and installation process again. If the previously downloaded Sound library has not been manually deleted and is still present on the machine, the download manager will simply copy the folder into the default library path. If the downloaded content is no longer available on the machine, the download manager will start the download from the beginning and the progress bar will
appear.
Once process is completed, a pop-up notification will inform you with a confirmation message.
(My underlining)
But I think that the wording should be "the download manager will simply install the file contents into the default library path."
If needed, I would rephrase your question as:
If I use "Reinstall Library" from the Product Manager
(a) will it download everything again (over 50+GB) from start?
or,
(b) will it see that the downloads folder already contains the files that were previously downloaded and will just perform the installation?
If the answer is (b), will it download any missing files?
Support's reply is correct but did not answer your specific question.
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Please not that, generally, quoting replies from Support is not permitted as the reply may confuse / mislead other users. In this case the reply is correct and applies to all users so I have not removed it.