Here's what worked for me.
Before I ever installed 5.0, I did a device backup. When I wanted to revert back to the BBM that was originally on my device, I went to Options>Advanced>Apps> and deleted the BBM 5.0.
Then, because I just have the stock AT&T OS, I did a Wireless Upgrade via the phone. Options>Advanced>Wireless Upgrade.
After it selected the appropriate upgrade, which was still the stock OS, I put a checkmark on the BBM and hit install. 20 minutes later I was good as new.
Re-installed the backup I did a few ddays before and all my contacts showed in BBM.
Not the only way to do it, but how I did it.
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