The answer is yes, you may downgrade/upgrade your rate plan at any
time.  And then the second answer is no, you do not have to extend your
contract.

I've read that if you want to change your plan AFTER 6 months of having
it then they will ask for a one year extension, but I'm sure someone
can clear that up for you.

*Looks for John Navas*  :)

In the first half of a contract, you can change to any rate plan w/ no
further commitment. In the second half, you will have to agree to 1 year in
order to get the "promotional" part of the rate plan (i.e. "Unlimited Mobile
to Mobile" on the current plans.) It works the same up or down, but you can
only keep roll over minutes equal to the standard minutes on the new rate
plan. (You can't change from 900R to 450R and keep more than the 450 of your
rollover balance)

Mark

As many times as I have changed my plan, I have never had to agree to any
new contract. The only time the contract ever is extended is when I purchase
a new phone at an upgrade price. I have added services and removed services.
I have changed my plans, both up and down, and never have I extended my
contract other than phone upgrades. I have never even lost roll over minutes
and that is changing plans with an excess of 3000 minutes in rollover.

Thank you very much for all of the information. Seems to me I will switch to
Cingular, because in Sprint, when I aded to start my Evening rate at 7.00PM
on December 2003, my 2-years contract was extended to Decewmber 2005. My
original contract was from May 2003 to May 2005. If anyone plans to switch
from Cingular to Sprint should be becareful.