WordPress 5.0 is expected to be released in August. It’s a big change that will break some sites that aren’t prepared.
Here’s my take on what ought to be done when a release date is announced:
suspend any automatic upgrades;
make sure a backup program is in place;
install 5.0 and test on a staging/development site before upgrading a live site;
install, activate, and test a "Classic Editor" plugin.
The big change in WordPress 5.0 is the vastly improved editor (called “Gutenberg”) and it won’t work on existing themes without revising them. The necessary revisions aren’t predictable so we can’t estimate even a ballpark cost for the necessary revisions.
Kitchen-sink themes like Divi and Avada will probably take it upon themselves to replace Gutenberg with their own page-builder, but they might be sacrificing some medium-term benefits — Automattic has big plans for WordPress and much of it is centered around maximum use of the new editor.
It’s a good time to start planning to re-do older sites, looking at completion in early 2019 when WordPress 5.x has stabilized a little.