You can go to
https://www.icloud.com/ and use the forgotten password link to get some details about the first one you setup, and hopefully recover the password.
When you go to switch them out, turn off each function individually in the iCloud settings for items you want to keep (i.e. Notes). It will ask you if you want to delete from the phone. Do NOT do that. When you're done with each item and ready to switch out, go ahead and Sign Out (all the way at the bottom of the iCloud settings) and then sign in with your original iCloud account you just recovered. This will sync your iPhone data to iCloud, and pull any existing iCloud information from before over to your phone (and transfer to your iCloud)