not that im aware of
the only thing that ma happen is data may be added if the phone requires a data plan
or depending on the age of the phone, it may not be compatible with your plan
You can activate an old phone both online or by dialing *228 from the device you want to activate, and there is no charge to do so. Just a couple of cautions:
If you are moving from a data to a non-data phone, and are on the tiered data plan, you may have prorating issues to deal with that could result in extra charges.
If you have an unlimited data plan, and move to a non-smartphone, you may lose your unlimited when you try to switch back.
Depending on the age of the old phones, they may or may not work with your current plan. (mdram4x4 mentioned this)
All that aside - we have several old phones that we have saved and activate from time to time or loan out to friends when they need a spare to carry them to their next upgrade. Activating another phone is pretty straightforward.