As others have stated, this is normal when doing an upgrade of an existing OS and all of its apps, data and settings. Some apps may not work with the new OS and others will be different in color or the way they are displayed, especially the native apps. This is why a full backup is always recommended prior to doing an update.
The other option is to install the update as new and install apps and data afterwards on a pristine OS.
I have read some horror stories about the Nougat update... as an AT&T user, my S7 Edge has the Nougat update available but I have not been prompted for it since I switch back and forth between an iPhone 7 and the Edge and I am not current on my Android security patch level (still on October 2016.) I have also heard that some Best Buy stores with the Samsung experience will allow you to flash your phone to get it up to date prior to the Nougat update.