Desire vsn Touch - Plan Confusion

[Krad69]

Here's a question, I am Tethering for free with my HTC TOUCH which is farely old. It's free cause I have a Data plan that no longer exist and it provides Internet surfing and tethering for FREE ,, preaty cool .. now ,, I have received in the mail today a letter from BELL (my provider) with the possibility to upgrade my phone, to get a new Intelligent phone.
I called them and asked,

- If I do, will I be able to keep my plan and use it with this new technology phone -
he replyed ..
- For the basics yes, but not for the Unlimited internet - so I asked -
Wy ? - et said
- Not the same network nor technology but you can pay 25$ monthly for 500mgs to surf and tethering - I said
- Wait, I am downloading gigs and gigs freely per month and you are asking me to pay more for a new tek phone with new gagdet and new network ? wy can't I keep my plan and adjut it to the new phone ?

Well.. can't do it,.. not gonna happend .... So my question is

IS IT possible for me to get the new HTC desire Z (or anyother smart intelligent phone) that I would be able to switch in the settings (when I need it) from WI-FI to the network that my HTC touch is using (I think it's ev-do or HSPA or something like that, I don't know) So I wouldn't have to pay more and I would be able to keep the plan I have and use it 100% unlimited INTERNET tethering etc... ???

Sorry for the loong post I as you can see.. I am new to this, newby to phone tek and stuff. I am seeking for answers before I get stuck with a phone for 3 years with only 500megs a month.
Regards
Krad