I would also suggest that you comment out any third party repositories that may have been added to the file /etc/apt/sources.list, or added in the directory /etc/apt/sources.list.d
It is probably easier to turn off any third party repositories in the /etc/apt/sources.list.d directory using the software manager. Choose 'Edit' then 'Software Sources'. These third party sources are often tied to the 17.x release of mint, or the 15.04 release of Ubuntu.
After your upgrade you can turn them on, one at a time, and see if there are errors or conflicts. If there are errors or conflicts, keep them commented out (or turned off in software manager). You can search to see if there is a Sarah based version, or an Ubuntu 16.04 based version of each third party repository. It may take some time for third parties to release new repositories after a major upgrade of Ubuntu or Mint.
If you have a great many third party repositories, it may be better to do a back up, full re-install and a restore of your data. This is the preferred method of a major upgrade for Linux Mint. Even with the third party repositories commented out, the software installed from them is for a different version of mint (and Ubuntu), and could cause instability.
Always do a full backup before attempting a major upgrade.