paypal subscription with IPN?

[phpsharma]

Hi all,

I need to integrate Paypal subscription package with IPN. My problem here is
1)The subscription system is already in use. for this i need to implement IPN.

Now my doubts are like this
1)can i implement IPN to already active subscription.??
2)how to modify exsting subscription.???
(ex :my package is presently offering initial amount:$40, monthly $20. Now i need to change this to $30 , monthly $10)

I have already gone through the paypal IPn.pdf tutorial.But i could not make it.

Thanks in advance.

sharma