No you will not be able to upgrade back to the $15 SMS Txt message plan in the future. It is a now or never option since the option has been removed from the available feature list. The only reason you could get it now is that is what you had before the upgrade to the iPhone 4S and all the recent confusion.
The only way to add a line to your account and hope to get this deal for your GF is to use the original iPhone loophole. You can add her line by using iTunes after you restore your old original iPhone as new, install a new Mini-SIM and follow the iTunes activation steps, adding the original iPhone Data plan with one of the combined SMS Txt message plans.
Does your GF currently have an AT&T account? If she is with another carrier you can even port her number to AT&T using iTunes and your original iPhone.
Once you get your GF working on the original iPhone then she can switch to a later version of the iPhone and the system will make the changes to her data plan.
Dave
Sneaky suggestion, Dave. And I LOVE it! G/F is currently on Verizon but plans to switch shortly. Many many thanks on this one!