How to keep my unlimited data and get the iPhone 5s on Verizon?

So my contract with Verizon ends this Saturday, 10/19 and I've had the iPhone 4s for the past two years. I'm grandfathered into the unlimited data and I don't want to lose it but if I upgrade to any smartphone, I will lose it. I'm on a family plan w/ my mom and sister. They both have basic phones and will NOT be upgrading to smartphones. Also, all 3 of our lines have been due for an upgrade since June. From reading online, I've figured I have two options:

1.) Buy the iPhone 5s at full retail price
2.) Do a cross/transferable upgrade

I have a couple questions and concerns about that. For the first option, if I buy it at full retail price, does that mean I'm month-to-month, no contract, so Verizon could take my unlimited data because they're not obligated to provide me unlimited data?

For the second option (and most viable option), is if I do the transfer upgrade first on a basic phone line then switch the phone over to my smartphone line w/ unlimited data, does that mean that my smartphone line is now under contract again or is it a month-to-month basis? I know that the basic line WILL be under contract.

Secondly, b/c my contract is up this Saturday 10/19 on my smartphone line, is it important to do this process before then or I could do it after the date? I want the iPhone 5s but it is backordered and I've read that you need to physical phone in order to do this.

Lastly, for the second option, how can it work when the iPhone 4s and the iPhone 5s have different SIM card sizes? Will this affect my ability to keep my unlimited data and the process?

Any helpful advice and input is greatly appreciated!

If you buy the phone outright you will be on a month-by-month basis, but they can't take away your unlimited data. Nothing about your plan will change at all except the fact that YOU can cancel it any time you want without a penalty.

If you upgrade the basic phone line to a smartphone, you can do what is called a "hardware swap" to move the new smartphone to your line. This will NOT restart your lines contract since you're not buying a new phone for that line, merely using one you already own (with it having been purchased previously on another line).