I was under the impression that you referring to a Palm device, because you posted you question in a thread concerning them. Palm smartphones use an entirely different mechanism than non-Palm mobile handsets do, so my experience doesn't even apply to yours.
You are likely correct: that a change to the plug-in architecture, specifically to the existing plug in which now supports your particular mobile handset in iSync 2.4, is probably responsible for your issue. That sort of problem does not exist with support for Palm devices in the latest version of iSync.
If you have a SONY Ericsson device, you should probably browse Julian Wright's support forum, here:
http://mobile.feisar.com/forum/index.cgi
Generally, when major changes to iSync occur post Mac OS X 10.4.6, Apple recommends that you back up your contact, calendaring and task records, remove any third-party plug-ins you may have installed, then update. They also recommend that you remove then attempt to add your mobile handset once again, and update any third-party plug-in which might be required to support it under the just installed release.
It's unlikely that you can correct a coding issue with the Agent or Plug-in likely responsible for this issue, though that is probably where the underlying issue lies.