My suggestion is not to do this. I would tell the users that you're making a change/upgrade/switch over to a new system. Tell them they'll be prompted for a new password the next time they login and have them set it up. It's a good opportunity to force everyone to change their password (better security practice). You don't have to tell them they can use their old password, so some will try this anyway and it should let them. At least some of your users, if not most, will change their passwords. If any of them have ever had their passwords stolen from another source, they'll be less exposed on your site.
I realize this isn't directly what you're after, but I encourage you not to go the down the path you're taking. Plus, it's simply easier, at risk of being slightly inconvenient for your users. At least they'll know what's happening to the degree they need to. It may even provide win you some cool points with your users for looking out for their best interest. If you tell them that you are, then they'll be less likely to get upset about it.