You can't migrate a catalog a second time, but you can export all the images from the migrated catalog to a new catalog in Classic (even just the trial version) and then migrate that to CC.
This was not my experience. I repeatedly migrated the same catalog when attempts to migrate hung or otherwise failed. I deleted my lightroom "cloud" database via the web app and then it let me migrate a catalog I had previously started to migrate.
In Lightroom Classic CC, go to Preferences and open the Lightroom CC pane. click on the Delete All Data button which will take you to the web app of Lightroom CC and allow you to remove all content. Make sure you have all of your images somewhere else, so you don't lose them all!
You can also chose to stop syncing individual collections from within Lightroom Classic to get back to a more traditional catalog structure in Classic before starting of the migration again.
Thank you this was exactly what i was looking for, i do have all photos and the old LR catalog backed up so it's all good. Just need to open that up again make some changes and then re-import to the new LR CC and all will be fine.
Also there seems to be a feature/bug where only the first keyword of every photo is migrated from LR Classic to LR CC. This is kind of a problem since i would like to keep all keywords. Ah well one step at the time.