How to Change iCloud Account Without Losing Everything

Hi. So, I've been using the same Apple ID for iCloud for a couple of years and I share that Apple ID with a couple of family members. I use iCloud across my 2015 MacBook Pro and iPhone 6s (with the latest software update). However, I want to change the settings so that only those two devices can access iCloud (and not allow family members' phones, tablets, Apple TV to access that iCloud account). Is there a way to do this without deleting from my MacBook/phone that's synced to iCloud and without losing access to all of the stuff backed up in that iCloud account? Like, could I change the login e-mail and password for iCloud ONLY (not for my Apple ID that we share for purchases) without losing access to the files backed up in that account or deleting them from my computer/phone?