How to change my Apple ID to be the same as my ICloud ID?

My Apple ID uses a gmail address suffix (@gmail.com) while my ICloud ID uses an icloud email suffix (@icloud.com). I set them up that way many years ago, but now I'd like to have just one account for both. The problems are the Apple ID (the one with the gmail suffix) is used to buy aps and itunes and I have a credit balance on that account. The icloud account (with the icloud suffix) is used for email and, naturally, for access to icloud for backing-up my iphone and such. Note that I use a pc (not a mac) and an iphone. What is the easiest way for me to change to one ID for both Apple ID and ICloud ID without losing my credit balance on my Apple ID (with gmail) or losing my iphone photos, back-ups, etc.? I would appreciate any help with this self-created conundrum.