I'd get a second opinion. I haven't read up much on the Alltel merger, but here in the Northwest we're finishing up the Qwest transfer. Early Termination Fees are waived for the Qwest customers switching over. As far as I know, they could potentially move one line over and keep the other lines on the Qwest account until Qwest Wireless finishes the merger completely.
It may be trickier with Alltel since they're not completely merging with VZW, and a portion of them are going to stay on Alltel (due to monopoly laws). I'd check VZW's website, there should be a place where you can check to see if your phone number is listed as part of the merger or not.