Releaser Notes for Base for V 5.3 say:
ODB files created by LibreOffice < 5.3 can be manually converted to LibreOffice 5.3 format by using Firebird 2.5 to convert the data to archive format, and replacing the database data within the ODB by the archive format version. To do this, install a stand-alone Firebird 2.5, and use its “gbak” tool to convert the file “database.fdb” to “database.fbk” within the odb file. Don’t forget to remove the .fdb file.
A STEP BY STEP TUTORIAL WOULD BE NICE! It is not obvious what all this “text” means. It appears that until I “convert” I cannot move to 5.3. (This is really poor quality work and documentation!)