Thanks a LOT for the answers.

It's now working as I wanted here:
http://nthinking.net/scripts/php5-3Migration/repDeprec.php

Basically as you said I had to separate the job in easier chunks.

As for the search and replace "paradigm", I do agree it's not the best solution...It is actually a very stressful way to go. I have been bumping into automatic code modification for few times now and the AST approach is definitely the way to go.
My solution can't deal with cases like:
split($myreg,$string)
where:
$myreg = '/'

But I guess this approach will do enough for what it is intend for...

Thanks Again!