syncing old iphone backup from itunes with current icloud

Bit of a complicated story, but around a year ago my iphone got stolen. I had made an old fashioned backup and I also used ICloud but the storage was full, so the photos and other content of the previous months were not saved on the ICloud. When I got a new phone, I found out the backup was password-protected and I didn't know the password anymore... So I just continued my life with the Icloud backup that I had with missing photos, contacts, passwords etc.

However, a while ago I found the password to the backup! I had an old phone laying around to restore the backup to see what was on there, and almost everyhting is on there, a relief! Now, my question is, if I log into my icloud on that phone with the old backup, can I sync those "old" photos and data with my current icloud? or will that mess up and overwrite the photos/documents etc I have created since losing my phone a year ago? I'd appreciate any advice!