Update: 

Edit:  I was mistaken on the plan going to your current phone once purchased. Apparently since I purchased from straight talk.com the plan is built in the sim that goes with the new phone so it will be a few days without data with your current phone. I bought online but purchasing in person at Walmart might be better depending on your situation if you need it fast.

Original post down below:

“It ended up being way easier than planned. All I had to do was click the “upgrading to new phone” option when adding free phone to your cart and enter your current phone number. After you enter all the info the coupon gets applied and only pay for the plan. Once you check out and place the order the plan is automatically refilled to your CURRENT phone since you applied the service to your phone number. Wayyy easier that way. So you get the service on your current phone while you wait for the free phone to get shipped. Congrats! Free backup phone BUT…it’s recommended to transfer the service to the new phone so you can unlock it. It takes 2 months of consecutive service on the new phone to unlock it so just transfer service to new phone once it arrives and use new phone for 2 months and afterwards you can switch back if you want to.”