Text Mesh Pro Upgrade

See the following sticky post / thread which outline the upgrade procedure.

In terms of changing Asset Serialization Mode, this is outlined in Step 2 and should be done on the backup prior to opening Unity 2018.3.

With regards to Collaborate, you should be able to disable it to simply change the Asset Serialization Mode and Visiable Meta Data. Once you have done so, re-enable Collaborate.