2 of 2 meanwhile the rpm version has changed

You neglected to add the default CentOS 7.x repos which is likely why you're running into your issue. Incidentally, you can fix this issue by grabbing yet another RPM:

$ rpm -ivh http://mirror.centos.org/centos/7/os/x86_64/Packages/centos-release-7-6.1810.2.el7.centos.x86_64.rpm

NOTE: You can find out what RPM to acquire using rpm to query to see what RPM provides a given file. In our case these files:

$ rpm -qf /etc/yum.repos.d/CentOS-* | sort -u

Once we re-install this RPM, it'll refresh our .repo files for the base repositories under /etc/yum.d. If we didn't know the URL where to download it, again, we can inquire to RPM to find out.

First take note of the name of the RPM:

$ rpm -qi centos-release-7-6.1810.2.el7.centos.x86_64 | grep Name
Name        : centos-release

Then search on pkgs.org for this shortname, which will lead you to the URL I showed above in the rpm -ivh ... command.

NOTE: The same steps can be applied for CentOS 7.3, or whatever version of CentOS you're currently using.