No. For subscriptions, you will pay the monthly price you agreed initially, new prices are for new customers.

Yes, you can upgrade to LTD for a discounted price in your account page. Discount will be the same as 1 month payment with the current monthly price.

You can cancel your subscription in your account page, or directly on your payment provider site. The 7 day money back guarantee is only valid for Life Time Deal.

Of course, your subscription will remain active until your current billing period ends. If you paid 1 month, you'll get 1 month. After that, you can keep your copy and do whatever you want, but your license will become invalid, you won't get plugin updates anymore and you will lose access to the Design Sets.

Since version 1.0.6, the Design Set is protected and only users with a valid and active license will get access to it. Use your license key to activate your plugin and get the Design Set and automatic plugin updates.
Also, don't forget to enable 3rd Party Design Sets under Oxygen settings.