Mike,
Thanks, but it seems the problem is not a "sync" issue/bug but instead a BB software one with the new system (didn't happen before upgrading). It is the mobile which after certain period of time (few days if so), by itself and w/o doing any sync, just goes ahead erasing past events. As I wrote be4, I have set the keep permanently all calendar events. No matter that, it keeps erasing them.
Weird!