You can already do that with replacements.
Just add simple replacement (not regex) in "REPLACEMENTS" tab.
Search for:
Code:
www.example.com/somepost123
Replace with:
Code:
www.abcd.com/xyzpost
Enable to apply to all new leeched messages if you want this to affect all websites OR later add that replacement for needed website(s) in "WEBSITES" -> select website -> "Replacements".
You can replace / remove word or sentence with replacements as well.
Check the FAQ about replacements:
http://leecher.freddy.lt/faq.php?expand=faq402