I "think" you'll have to activate it on the line you upgraded. Once that is done you should be able to switch phones.
This is what I've done in the past and will do with my iPhone as well -
- Activate iPhone on the line you upgraded
- then turn it off and activate the old phone back on the upgraded line
- you should then be able to activate the iPhone on essentially any VZW number
VZW needs you to activate so they can account for the upgrade you used and ETF you if you drop service