OK, folks, here's what I did. Worked like a charm. A couple of steps were confusing, but it wasn't hard to figure out, and I did it all online...
* Ordered new GS4 for my daughter's 2 GB line, but using eligibility from my unlimited line
* Receive new GS4 today
* Installed SIM and battery that came with it into the GS4, as well as SD card from my old phone (Thunderbolt) from my unlimited data line
* Turn off daughter's phone (the one from the 2 GB line)
* Activated the new GS4 on 2 GB line by turning it on and going through activation process
* Turned off new GS4
* Re-added daughter's phone back to her 2 GB line on VZW site as follows...
- Logged into VZW.com
- On the My Verizon menu at the top, chose Activate or Switch Device
- The next page had all phones under my account shown. I selected the GS4 which was shown as being assigned to my daughter's phone number. Clicked Next
- The next screen asked if I wanted to replace the GS4 with a phone that had already been previously activated on my account, which I select since my daughter's phone had been on this account before I replaced it with the new GS4. My daughter's phone was in a dropdown list, so I selected it.
- Next screen instructed me to call *228 option 1 to activate my daughter's phone back on her 2 GB line.
- Turned daughter's phone back on. Dialed *228, selected option 1, went through voice prompts on phone. Phone rebooted itself when done and her phone was back on her 2 GB phone line
* To add the new GS4 to my unlimited line, went back through above steps, this time selecting (on the Activate or Switch Device screen) my old Thunderbolt that was still assigned to my unlimited data line, then took these steps...
- (This is one place where the Web site didn't make sense) After selecting it and clicking Next, the following screen was different. It showed the new GS4 and asked if this was my new device (to replace my Thunderbolt on my unlimited data line), but didn't give me a Yes or No choice.
- COPY DOWN THE DEVICE ID AND SIM CARD # THAT IS DISPLAYED HERE... it will make the rest of the steps a bit easier.
- This page also asked for my Billing System Password. This is also sometimes called your PIN. It is NOT your My Verizon Web site password. It's 4-5 characters. You can call VZW to have it reset if you don't know it. After entering it I clicked Next.
- The next screen was more like what I saw when re-adding my daughter's phone to her line, except this time I selected that I was adding a phone that had NOT been on this account before. Yes, the new GS4 had technically been on my 'account' before, but I think it means 'on this phone line before.' I had to provide the Device ID (I'm pretty sure - I can't go back through the steps and check), and the SIM # (definitely), so be sure you've copied those in the above step.
- Once I stepped through and completed this process, I turned the new GS4 on. ***NOTE: It took 15 or so minutes before it was accepted by the VZW network. I was afraid something was wrong and that I'd have to call VZW and get stuck in CSR hell, but don't panic. It just takes some time. While it was getting set up, I went back to the Activate or Switch Device screen where the process started, and the new GS4 was shown on my unlimited data line, where I wanted it, but it was grayed out and not selectable, with a note under the name saying that the request was processing, or words to that effect.
* Now my new GS4 in on my line and has UNLIMITED DATA (well, at least until VZW pulls the plug on all unlimited data plans... but I'll take it!)
Anyway, that's how it went down. No phone calls to VZW. No swapping of SIM cards. Certainly worth the effort to keep my unlimited data plan!
Let me know if you have questions!