Of course you will be able to access your data!
Storage: you’ll need to comply with the storage quota of your new plan. Meaning, in case your existing stored data exceed the new plan’s storage quota, you’ll need to remove some of your files to be within the new storage quota.
Features: you may lose specific features and/or have lower quota on features available (e.g. advanced dashboard, custom-locations, custom-categories, sharing, etc). Indicatively, if you are a Plus subscriber and downgraded to Free, you may have 300 alerts configured but only 30 can be active, in compliance with your new Free plan. All your already configured alerts (300) will remain intact but idle thus no notifications will be send. Should you upgrade your plan, the idle alerts will be reactivated, in line with your new plan’s quota.