Please make a complete backup with TWRP of your current ROM and data, in case anything might go wrong!
The builds are signed with release keys!
If you want to install over an existing ROM without losing your data, then you first have to install the ROM which ends with "_migrate", it will remove all existing key info at boot time. Do not stay with this "_migrate" ROM but install a signed ROM without "_migrate" soon afterwards.
For more information please read this information from LineageOS: https://wiki.lineageos.org/signing_builds.html
Or you do a factory reset and start with a non-"_migrate" ROM. (You will lose all your data!)
Further updates to this ROM can be done directly without data loss, no special migration necessary.