Yes you can,
Once you have purchase a active subscription you can see remaining plans as ‘Change To’
By clicking on the ‘Change To’ button new popup will be shown by Google play, where new plan details & validity will be shown, upon subscribing new plan will be activated.
And coming to price difference between plans,
The subscription is upgraded or downgraded immediately, and the user is charged full price for the new entitlement immediately. The remaining value from the previous subscription is either carried over for the same entitlement, or prorated for time when switching to a different entitlement.
Note: If the new subscription has a free trial or introductory offer, the user is charged $0 or the price of the introductory offer, whichever applies, at the time of upgrade or downgrade.