You may be aware that the CentOS community has released CentOS 8 on September 24, 2019. This new release is based on Red Hat Enterprise Linux 8 and holds all features of Red Hat Enterprise Linux 8.
READ: CentOS 8 – New Features
Red Hat provided a path to upgrade from RHEL 7 to RHEL 8 for its customers who are interested in upgrading RHEL 7. Customers who are running Red Hat Enterprise Linux 7.6 can upgrade their system to Red Hat Enterprise Linux 8.
READ: How To Upgrade From RHEL 7 to RHEL 8
Like Red Hat, we expected the CentOS community would provide some option to upgrade from CentOS 7 to CentOS 8.
Unfortunately, the tool leapp that helps to upgrade from RHEL 7 to RHEL 8 may not be available for now, and it seems the CentOS community has no plans to support this package for CentOS as per TrevorH on the CentOS Bug Tracker.
In his words,
There are no plans for the CentOS core team to support or package leapp but if there is sufficient support from the community to provide copies amended for CentOS then they could be released as part of a SIG. However, given the current lack of support for the preupgrade tool to migrate from CentOS 6 to 7 and the total lack of response to all calls for volunteers to package that, I would not be optimistic about it happening.
From: CentOS Bug Tracker.
The preupgrade tool to upgrade from CentoS 6 to CentOS 7 lacked maintainers, and it no longer works.
READ: How To Upgrade From CentOS 6 to CentOS 7
So, we afraid to say we may not be seeing any options to upgrade from CentOS 7 to CentOS 8 near future. As a recommendation, we ask you to do the clean installation of CentOS 8.