To answer your first question, see this article:Change your Apple ID - Apple Support(Basically, you just want to follow step 1 in the "Change your Apple ID" section. You will be creating a new Apple ID, not changing it.) Then see:Create and start using an Apple ID - Apple Supportand especially the last "If you want to sign in with a different Apple ID" section (and the link therein).For your second parenthetical question, be aware that Apps & Songs purchased through the App & iTunes Store are permanently tied to the original Apple ID and cannot be shared or transferred to another Apple ID. However, you could use Family Sharing with your sister (and still have separate Apple IDs) to share those purchases:Family Sharing - Apple SupportSet up Family Sharing - Apple SupportShare purchased content with Family Sharing - Apple Support

Possibly, but it’s gonna be a pain and there are several things that may go wrong. Is it absolutely necessary? If so, make a backup and dive in.

Just move your data first… Notes - move your notes to the “On My iPhone” section. Messages - turn off messages in the cloud and choose the option to store them locally. Photos - turn off iCloud Photos and choose to download them to your device. Even if you don’t do that first, when you sign out it should give you the option to download your data. Once it’s local on your phone sign out of iCloud and sign in with your new AppleID. There are also 3rd party apps that will let you archive your stuff and move it back. iMazing is probably the best.