Comments - unable to upgrade from MariaDB 5.5 to MariaDB 10.0 using YUM
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There is a known bug MDEV-5986 which is the reason for the sed error, it will be fixed in the next version. But even after this, it might not always be possible to do a major upgrade (e.g. 5.5=>10.0) automatically, in some cases it requires the manual process as the comment describes. Please follow the instruction provided in the output.
Uninstalling the old version will not remove your data, but as for any upgrade, previous data backup is strongly advised.