I just tried the new Individual Tracker Options to have a tracker URL replaced by a different one, but either I did something wrong or it's not working.

I just wanted to replace a string, so I used:

(original URL)
replace_in_new: (new URL)

Without the parentheses, obviously. Although I did try with the parentheses TOO, just in case. And I tried both with partial strings and the full tracker URL. However, the tracker remains unmodified when I add a new transfer.

Am I missing something, in the syntax or otherwise ? Please advise, thank you.

You've got the syntax right.  There's a few bugs in the tracker options that are likely responsible.  We're getting those fixed up right now, and the next release should work much better.  It should be ready very soon.