I need your help and/or suggestions to solve a (maybe uncommon) task.

I searched a lot on the web and  I had some success using a tool called fbclone but

this is definitely not a solution (due to extreme slowness of the conversion, many days,
something must be wrong with this software)

I have a large Firebird 2.5 DB (a couple of tables have few millions records) to convert from WIN1251 charset to UTF8 charset.

Is there a way to do efficiently this kind of conversion? (preferably not using a intermediate text sql dump, that would not applicable because the size of the DB).

I'm using Firebird on a linux machine, and I really like to do the job using a bash script,

but I can fallback also using a remote Windows pc.

Thanks for any suggestions.

Best regards.