Official migration guide is now available on
https://docs.turris.cz/geek/tos3_migration/.
Please be aware that this is still experimental and problems might be present. You
should read the documentation in its fullest before you attempt migration.
You can also check Gitlab issue about migration to get idea about features that are covered by migration.
If you are going to perform migration on Turris 1.0 and Turris 1.1, be aware that Turris OS 5.0 provides only experimental support for these models.