I have developed a database upgrade script which allows to bring older installations of phpwcms to UTF-8. It's in an early stage but it seems to work as expected for ISO-8859-1 (tested a lot with it, other charsets should work too).
It keeps serialised arrays intact and is also able to fix broken serialised arrays (as far I can tell).
It's a one time process taking the old tables and duplicates it with a new table prefix and changes charset to UTF-8 and collation to utf8_unicode_ci — utf8_unicode_ci is more precise compared to utf8_general_ci. Then all rows from the old tables are selected and copied into the new tables. During this step a parser sanitizes all text/binary field values and convert serialised values too.
When finished all you have to do is change phpwcms' config and hope all went well
If you like to get your DB converted let me know.
Cheers,
Oliver