If you swap a SIM card from Blackberry to Iphone will the IMEI of the Blackberry remain on the account until you call ATT to have it swapped over?
Your bigger issue is not the IMEI's but ensuring you have the right data plan associated with the phone you are using. Even though the price is the same, you need to have an iPhone plan active when you use the iPhone. It ensures proper billing and that the associated iPhone features are available to you and any Blackberry features that you no longer need are turned off.
For service or support questions including existing order status, call: Customer Support: 1-800-331-0500
Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/