If upgrading your account to a higher plan, your new plan will begin immediately. Unused trips on your current account do not carry over and will be lost. In this case, you can avoid losing trips by making sure you've used all your allotted trips before switching to a new plan.
If downgrading your account to a lower plan, the new plan will start on the next billing date. You'll be able to use any unused trip allotments before the next billing date.
If changing to a custom plan, contact your Customer Success Manager for assistance.