If I buy Unity iOS Pro from a webs tie that is offering a discount, can I update from 3.4.2 to 3.5 beta without loosing the Pro version?
HOLD ON THERE!
The only website authorized to sell Unity (other than Unity, obviously) is Studica, a store for selling things to educators and students.
Assuming that you are not buying this for your school and you are a student, Studica offers 2 versions: an educational-use only version, which costs $100-$300 and the student-commercial version which is 50% of the normal Unity price. The educational-only version may only be used in an educational environment and may not be used for commercial purposes. Also, it is “not upgradeable”, which might mean that you can’t upgrade from 3.x to 4.x or might mean minor versions (like from 3.4 to 3.5).
Since that is probably not the version you want, you probably are looking for the student-commercial version, which again, is 50% off the normal price, but it is upgradeable. However, you only get one license so you can only put it on one computer (as opposed to the normal license which you may put on a maximum of 2 machines). It also has a few other restrictions.
However, if you are not a student (or an educator looking to get Unity in your school), there is no place for you to buy a discounted version of Unity (that I know of). Do not trust any website that is offering discounts unless it states on Unity’s site that they are an authorized distributor!
So yes, the student-commercial would be able to upgrade from Unity 3.4 to 3.5 beta and beyond.
Yes. I am buying it from studica. And if I can’t update, nor publish my game, I guess I’ll buy the student commercial version of unity from the website. 
I’ll go see how much it costs.
Thanks for the answer! 