Is there a way to make rsync not copy over existing files BUT will update the date stamps?

I'm trying to set up nextcloud. It's REALLY slow to do a large initial backup, and I've been reading Rsync'n large data sets first, then telling Nextcloud to scan for new files is a much faster method to get it up and running. So I rsync'd over 450GB of files to Nextcloud. Got nextcloud to scan for new files, took many hours (12) but eventually finished. Then I go to set up the nextcloud client and OMG, it's downloading all of the files again (estimated time, 6-20 days), why?!

Turns out when I rsync'd over my files it updated the date stamp of the files and that is all it takes to make nextcloud trigger a transfer.

So how can I fix this? If I have to redo this whole process I am willing to, but if there is a quick way I can make rysnc fix this I'd rather do that.

Thanks so much in advance!