Going from 5 to 6 might call for a few more in depth script twists than 4 to 5!
As per the manual those functions
eregi, ereg, ereg_replace, eregi_replace
are deprecated since 5.0.3 and will be removed from PHP6
So in most cases if you use any of them you will need changing it by a preg_match() or a preg_replace()
Make a full scope search for the ereg family
and you could be in for a surprise
If you use PMA then hope for an updated PMA without eregi
If you use class.phpmailer.php
use the newer one (most recent is 5.1)
If you Flash library uses write.id3v2.php
then we must hope for a new release, so far it still uses eregi
Do you have more on the topic?