Migration Assistant and ~previous system configuration files

I successfully ran Migration Assistant during the process of migrating a Mac on Mavericks to a new one on Yosemite.

On the source Mac, I made a few modifications on system configuration files for security reasons. These files are within the /etc directory:

/etc/services
/etc/sshd_config

After the run of Migration Assistant I found some of my modified configuration files under the same name but with a suffix of ~previous and my file was replaced by a new one without my modifications:

/etc/services
/etc/services~previous
/etc/sshd_config
/etc/sshd_configd~previous

I also found some other files with this ~previous suffix on files I didn't modify:

/etc/group
/etc/group~previous

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Is this mechanism a kind of backup of Migration Assistant to protect a new system against potential incompatibility within configuration files?

Is the resulting new file the source one from Yosemite or a merge between the source one and my modified one?

Where is the managment of these ~previous files documented?

How should I deal with them?

(I already hunted the Knowledge Base about these files).