Found this, which seems to indicate, yes it can be done.
https://linuxacademy.com/blog/linux/centos-7-upgrading-from-centos-6-x-in-place/

On Mon, Nov 27, 2017 at 2:20 PM, Ed Greshko <ed.greshko@greshko.com> wrote:
On 11/27/17 21:34, Ger van Dijck wrote:
> Is there a way to upgrade Centos 6 to Centos 7 (32 Bits) in the same way as I can
> upgrade Fedora 26 to Fedora 27 with the utility "dnf system-upgrade" ?


A quick Google search seems to indicate that upgrade from 6 to 7 is problematic at
best and disastrous at worse.

Google search term was "centos upgrade 6 to 7".

But you may find more help by going to https://www.centos.org/forums/  or
https://wiki.centos.org/GettingHelp/ListInfo

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