Thank you for reporting this. I expanded instructions in the first post for Turris 1.x. I missed that we have check in Turris OS 3.x release but it is still possible to install tos3to4 package manually and trigger migration that way.
Edit: Please do not attempt migration on Turris 1.x until Turris OS 5.2.1 is release (which should be in short order). I manage to discover bug that causes pretty important part of migration being skipped on Turris 1.x.