Removing unneeded changeSets is easy because Liquibase doesn’t care about DATABASECHANGELOG rows with no corresponding changeSets. Just delete out of date changeSets and you are done.
Just use the checksum in the error message in the validCheckSum tag to mark that you know it changed and the new value is OK.
Sometimes it is easiest and safest to wait until all your databases have the columns and then modify the changeSets to avoid the preconditions.
If you end up finding that it will work best to completely restart your changelog, see starting Liquibase with an existing project which describes how to add Liquibase to an existing project