Deleting a pricing package

Hi,

I would like to be able to create an offer that if customers list they can have a reduced listing price but I want this to only be available for a set time i.e 1st July to 1st December
But I want it to be a sliding scale:
For example
* If you list in July you will get the reduced price for 6 months
* If you list in September you will only get 3 months at the reduced price

And so on until the offer ends and then all those customers on this plan will automatically be upgraded to the new higher price package.

Is this possible ?

I can only think of doing it by creating two plans one at the lower price and set it to expire after 6 months and click the downgrade to the new plan but have this as a higher priced package. However this will mean that everyone that signs up has 6 months reduced price ! no matter when they join.

If this is the case that everyone that takes up the offer in this time has 6 months at a reduced rate what happens when I delete the package in December so no one else can join will this affect the users that have already signed up ?

Thank you Kate