Yes, you can. For example, if you're subscribed to a Free license you can easily upgrade to a Plus or Pro license. If you have a Monthly Plus license, you can easily switch to a yearly Plus or Pro. At payment, any unused budget of your current license will be subtracted pro-rata from the price of your new license.
Please note that your current subscription needs to be active in order to instantly change your license. If you already canceled your subscription, the changes will only take place when your canceled subscription expires. You can reactivate your subscription at any time via your Profile page.