(keitai-l) Changing to i-mode - 4 weeks required?

Hi Everyone,

This isn't technical or marketing related, but here goes. What
is the reason for NTT requiring you to have had your non-imode
cellphone for at least 4 weeks if you want to switch to i-mode?

I finally bought an i-mode phone last.  The shop clerk asked me
(and was asked by NTT on the phone) if I had had my phone for
over a month.  I was asked the same thing by an NTT DoCoMo
store clerk two months ago when I tried to buy an i-mode phone.
If I hadn't used my current phone for more than 4 weeks, the price
would go from 10,000 yen to 40,000 yen.

Is this just NTT's way of trying slowing down the pace of current
users switching off to i-mode?
 
Thanks in advance,
Jon