Yes, you can change whenever you wish. 

To change to an 'Unlimited' option
1) Login to your account.
2) Click 'Cancel' beside your 'Pay As You Use' Option on the Summary Page.
3) Your number will now default to a 'Free Account'.
4) Click 'Upgrade' on the Summary Page, you can then select the option you wish to change to.

Note: You will be charged an 'activation fee' but the total amount will be added to your call credit. You will not loose service or be charged penalties at any time.