User can modify your paypal amount, but some restriction are there.
In Standard account, you can use "modify" html variable to change subscription amount as below.
0 – allows subscribers only to sign up for new subscriptions 1 – allows subscribers to sign up for new subscriptions and modify their current subscriptions 2 – allows subscribers to modify only their current subscriptions The default value is 0.
Following link will hel you more, https://cms.paypal.com/uk/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_subscribe_buttons#id08ADF500NXK
you can not modify often your amount, you can only increase the subscription amount by 20% for 180 days.
but if you move to paypal payflow($30/month), paypal itself provide API to communicate with paypal service. its very flexible to modify, cancel subscription.
paypal standard - user cannot cancel subscription their self if they dont have paypal login. paypal payflow - user can able to cancel their subscription.