Changing TabBar on-the-fly?

[lumala]

Hi guys, I'm quite new in iPhone Programming, but I'm trying to do my best cause I have some skills in OOP.

I'm creating an iPhone app based on a four element Tabbar. I would like to use other functionalities already implemented by a friend of mine in his own iPhone app (a three elements TabBar app).
Since I would like my app's users could "switch" to old application (3 tabs) by pressing on one of my app's tab, my question is: is it possible to do this, maybe by "popping out my tabbar" (4 tabs) and "pushing in the old tabbar" (3 tabs)?

For a better problem description, we could say that I'm trying to implement a "new mode" and "old mode" in the same application.

Thank you in advance.

P.S. I'm using Xcode 3.2.2 on a Macbook Pro coreDuo and 3.1.3 iPhone simulator

Luca