Change svn:mergeinfo property on an old revision of the repository (history) which has an invalid syntax

Hi,

unfortunately trying to svnsync a repository i discovered that there is
an invalid revision in the repository having a wrong svn:mergeinfo
property, instead of

1280-1282 it has 1282-1280

"Commit rejected because mergeinfo on '/branch' is syntactically
invalid"

The client we're using t sync is version 1.9.7. The source server is
1.6.17.

What's the best way to fix only those broken revision? Is there some way
to fix this property in the history? Or is there some way to fix that on
the fly? Any suggestions welcome.

kind regards

Torsten