Hi. If you try to add or open a vault format 6 with Cryptomator 1.5.x, this will be detected an Cryptomator will ask you to start the migration process. Although there are reports that „on demand files“ like google file stream or OneDrive on demand just need more time (but will work), I personally recommend to have all vault file fully synced and offline available for the migration. (Let me point out that this is a personal „feeling“ thing. Technically there’s no need for that. See here: Migration from old to new vault 7 (files on demand) - #11 by crypto1)
If there is an error during migration or the migration is interrupted for whatever reason, the vault will not be lost. The detection wether a vault was migrated or not is file based, so the migration process can be resumed.
If you do not have backups of the files in your vault, I strongly recommend to create one. This has nothing to do with the migration, but with the fact that important files always should have a backup. No matter if they are stored in a Cryptomator vault or anywhere else.
I’m using the new vault format with desktop and iOS since the first betas and never had any issues. So from my point of view there’s no need to wait with the migration.