Posted 08 June 2017 - 04:23 PM
In general, it should not mess anything up.
The closest might be in syncing contacts and calendars and reminders. It should merge the contacts and is usually good about not creating duplicates, but you might end up with a few duplicated if you have made changes to say a contact on both devices before the merge occurs.
If you want to be on the safe side, then you could delete all the contact and calendar entries on the old phone before you log into iCloud. I don't think you need to do that, however. I doubt you will have any issues.
FWIW, you DO NOT need to fully sign into iCloud in order for your iMessage messages to go to both phones. iMessage (and FaceTime) both have their own sign in process separate from iCloud. So, you can actually use a different AppleID for iMessage (and FaceTime) than you do for iCloud. In fact, there are four different AppleID sign-in options...one for iCloud, one for iTunes & App Store, one for iMessage, and one for FaceTime. You can use the same AppleID for all 4 at one end of the spectrum or use a different AppleID for each one (i.e. 4 separate AppleIDs) at the other end of the spectrum...or something in between. And actually, there is more in that you can add additional iCloud accounts for email access to mail if you have more than one iCloud account that you use for email.