I want to try public mobile, i'm curious if I sign up for 1 gb for 30 days will i be able to change throughout the plan if i'm using more than i think i will?
PM is a prepaid service, if you renew / change plans in the middle of your cycle you are not getting any refunds for the unused portion of your plan.
Say you are using your full 1GB in 12 days you can start a new plan then, it's the most cost economic. But if you have 600MB left on day 12 and decide to change to another plan on day 13 you will loose those 600MB without a refund.
Another aspect to consider: Say you are happy with the coverage and decide to change to one of the bigger data buckets at the end of your cycle while watching the usage on your current alotment, you can future date that plan change in your self-serve account.
This approach is highly recommended if you are interested in one of the bonus data plans or a promo plan because those can disappear essentially without notice. But if you future date the change to the according plan while it's available, you will still get the plan even if it's not open for new sign-ups anymore when your renewal date comes around.