Yes, you can change the start time or date of any plan you've purchased as long as the plan hasn't started yet. In order to change the start time or date of your plan, login to your AOW account online, click on the SIM the plan is scheduled on, and click Reschedule beside the plan you'd like to modify.
You can choose the time zone you’d like your plan to operate in, so make sure you select the appropriate time zone for your destination.
You can reschedule your plan up to 6 months in the future.
Plans purchased through an Apple SIM on iPad start as soon as the plan is purchased. There is no option to reschedule because the plan will start immediately after purchase.