B. Transfer my upgrade to son's phone (line#3) get a comparable phone and then switch the upgraded phone from his line (#3) to mine (#2)?
Would line #2 keep unlimited data? Buying the subsidized smart phone requires a data plan, right? But, he is too young to have data and the old phone would not support it anyhow, can we just transfer the phones, cancel the data plan and keep the phone?
Yes. This has been brought up quite a few times.
1. Transfer your upgrade to the basic phone line.
2. Order the new phone you want on the basic phone line.
3. Activate the new phone on the basic phone line (will add 2GB plan to that line temporarily)
4. Re-activate the basic phone on the basic phone line (will remove the data plan)
5. Activate the new phone on your line, which WILL KEEP unlimited data.