jen716 wrote:
Thank you, that's helpful. And just to confirm, you can do an Assumption of Liability with the contract from an individual plan to a family plan? And then get the discounts associated with a family plan?
If you are currently eligible for upgrade, and your sister is willing to add your line to her family plan and has a line eligible, then as Ann said, upgrade first, then have her do the AOL. We actually did just this a few months ago, and were able to do it all in one transaction at a corporate store, with no problems at all.
My daughter had a single line, and we had an open slot on our family plan, and she wanted to save some $. She also wanted an iPhone and was due for an upgrade. We went to the store, got her iPhone upgrade and then I did an AOL for her line, keeping her phone number. She no longer has an account, and I have her line added to my family plan, she just pays me for her portion, which is less than her individual plan was, and she enjoys our unlimited texting. 
As said, don't cancel your plan - you'll lose your number. You will not avoid the $30 upgrade fee in any case. If you cancel and add a line, you'll have a$35 activation fee and a new number. The AOL will keep your same number, and there's a $30 upgrade fee.
Hope this makes it all clear.