How many of you have an old React app built on an older version that needs to be upgraded? How urgent is it to get this done?

I recently had to make modifications to an existing React codebase written many years ago and had difficulty re-acclimating to older versions, and struggle d to use the design patterns of the time.

I also had to constantly struggle not to start the down the path of upgrading code where it wasn't necessary, especially upgraded the React version since that would be a very bad idea given there was no reliable regression testing in place for it.

I was wondering what the market is for this type of service and if people just bite the bullet and rewrite it in an upgraded language, or sunset the application altogether?