Whether you keep the time you've already paid for depends on the subscription change you're making. In general, past payments can't be applied as credit toward a different subscription. For the full rules on billing cycles and cancellations, see our Refund Policy.
If you have an active yearly or monthly subscription and purchase a Lifetime subscription, the amount you've already paid for your current plan can't be applied as credit toward the Lifetime purchase. Lifetime billing takes effect immediately, and any remaining time on your yearly or monthly plan isn't credited or refunded.
For details on the Lifetime plan and its current price, see Can I switch from yearly to lifetime billing?
If you switch from monthly to yearly billing, the change takes effect at the end of your current billing cycle. You'll keep the time you've already paid for, and your yearly subscription begins once your monthly subscription ends.
The same applies in reverse. If you switch from yearly to monthly billing, the change is scheduled for the end of your current yearly term rather than taking effect right away. You'll keep your yearly access through the end of that term, and monthly billing starts once it ends.
Switching your subscription is simple and can be done directly from your account page. For step-by-step instructions, see How Do I Change My Billing Plan?