With the advent of Umbraco 8.1.0 it is now possible to upgrade a site from a previous version to Umbraco 8.  However, this is by no means an automated process. The new upgrade feature in 8.1.0 works similarly to previous Umbraco upgrades - they upgrade content stored in the database, but don’t provide provision to upgrade custom code written in Templates, Partial Views or Controllers.

Umbraco 8 is a substantial breaking change from Umbraco 7, and so the effort needed to upgrade an existing site shouldn’t be underestimated.  Critically the code used by Umbraco Developers to create Templates, Partials and Controllers undertook a substantial breaking change. Some of this change is not complex, but it will require all code to undergo changes in order for it to work with Umbraco 8.

With this in mind, it's worth considering if there is a business case to undertake the upgrade.