From a mobile device its settings > iCloud > storage > change storage plan and from a Mac it's system preference > iCloud > manage > buy more storage, you will need to be logged into the appropriate account to do this.
You won't be able to use iTunes content unless you continue to use the old account for iTunes (if you use automatic downloads or iTunes match, you can't keep switching either), whether or not you can still access photos depends on how you currently manage them and how many there are. What size is the library, do you use photo stream, iCloud photo library or both.