That's B.S. - if I'm able to get the $59.99 plan, then so should you. However, one caveat, my CSR kept me at my pre-BlackBerry plan (but we added BlackBerry Unlimited Data) until 10/27; which is when I'll officially switch over to the $59.99 BB plan. So, hopefully, the system won't deny the switch. Oh, my CSR also offerred to switch me immediately to the $59.99 BB plan but told me it'd make my current monthly bill appear all whacky so I opted to just wait until the end of my current bill cycle. So, it appears I should be able to switch to that plan even though I'm an existing customer.
Regarding the link to the $19.99 BB Unlimited Data add-on, I can't send you a link as I was only able to find that information after going through the process of signing up with Tmobile as a new customer. So, in order for you to view it, do the following:
1.) Choose a national rate plan by adding it to your cart.
2.) Then, choose the 7100t phone by adding it to your cart.
3.) Then, you'll see a list of optional add-on plans. In this list, you'll see the BB Unlimited Data add-on for BES and BWC ($29.99 & $19.99).