I always rationalized it this way:
There were going to be people resurrected who had never been tested.
There were possibly going to be children born in Paradise who had never been tested.
After a thousand years in Paradise, people who survived the first test, might become "complacent" and think things were better in the "good old days" (remember how the Israelites didn't appreciate the wonderful manna they were getting, and wanted to go back to Egypt for the figs and cucumbers and melons??) so these ones would need to be reproved.
Finally, there was the idea that at the end of the 1000 years, everyone would have attained perfection again, and would be in the same state as Adam was in the Garden of Eden. According to the WTS, it would be appropriate to put perfect humans to the test the same way God put Adam and Eve to the test in Eden.
But now that I'm reading your post, I'm thinking "holy crap, this God is just NEVER satisfied with what we do, always having to put people to the test, over and over and over; what a JERK!"
Love, Scully