Everyone seems to get scared when they see their plan don't match their current one at all. I did it just now to confirm and I do not have a $99 voice plan...
Just go to Add to Cart and Continue and you'll see this...
Notice "No Voice Plan Changes"
Also, yes you can keep your unlimited data. It automatically keeps it without even bothering you to change it!
PS: OMG I love the selection screenshot shortcut on Mac's...never used it before haha

Just go to Add to Cart and Continue and you'll see this...

Notice "No Voice Plan Changes"
Also, yes you can keep your unlimited data. It automatically keeps it without even bothering you to change it!
PS: OMG I love the selection screenshot shortcut on Mac's...never used it before haha