Regarding Pausing the subscription, yes you can pause it for upto 90 days by suspending your subscription in Payments>Manage Subscription and toggling "Subscribed" switch to off.You won't be charged any longer & can keep your number & plan suspended ...
You can start your new subscription right away or at the end of your billing cycle. My understanding is once you switch to a new plan you can change to any plans available at the time, if your current plan was a promo it may not be available when you...
Hopefully you were able to change to this plan, if not it is available till 4th March. Also you can choose to activate new subscription at the end of your billing cycle so that you are not charged again.