I did the same thing earlier this summer, except I swapped out my old Ritchey OCR rims for Kinlin XR-270 rims. In my case, the ERD was off by a couple of millimeters.

If the new rim has an ERD that is 3mm less, then the difference in effective radius would be half of that, which means that your current spokes will be roughly 1.5 mm longer than necessary.

If it were me, I'd check to see how far the spokes are embedded into the spoke nipples, in order to judge whether or not the extra spoke length is going to cause any problems. I doubt that it will.