Hi Aishin,
I think that there is no codepage conversion between ISO8859-5 to ISO8859-1, but in case you didn't use the extended characters of the character set, you should be ok.
You can do that using 'proutil convchar'; take a look at the KB #19912 - 'Converting an Existing Database to UTF-8' for more information how to do the database conversion.
In case you encounter more problems, you should take the thread on the <A HREF="/forumdisplay.php?s=&forumid=40"><B>I18N</B></A> forum.
HTH,
Dan