Will entering my iCloud now overwrite all my contacts, calendar, etc from a fresh phone that's been in use since the 5s release?

Question

When I first purchased the phone the gentleman, just activated my phone handed it to me and that was it. Blank empty bran new iPhone 5s iOS 7 activated. I was stupid enough not to enter my iCloud info immediately but on that day we were rushing around to find the iPhone 5s in stock anywhere Apple had them for my brother so iCloud became a second concern.
But now I'm stuck with the problem of that the phone has collected contacts, pictures, mail, bookmarks, since the release date. I recently updated to 7.0.4 through iTunes I setup as a new phone, as it was the first time hooked into a computer. It took my AppStore account and found a old iCloud account associated with it that I don't use. I told the update not to use iCloud, even though it entered in all the info. I was able to delete this info and I'm back to the sign up in settings for iCloud.
If I were to enter in the right iCloud information now will it purge the data on this phone that's collected data or will it just simply merge the phone and whats on iCloud like the contacts, calendar events, reminders, notes? I have important things that I've collected on my phone and I do not want to lose it.
Will iCloud overtake the phone or will it merge what's on my phone now with what's on the iCloud account to bring them together?
Thank you,
Brandon

Answer 1

Maybe I should just backup this phone before trying merge everything from my icloud with what's on my phone.  I just can't lose the data on this phone I've collected. I just want my iPad and iPhone back to the way they always were with icloud.