At the very least, before upgrading make sure that any plugins, templates and the core being used is compatible. If not, reach out to see when or if it will ever be compatible. Sometimes a good developer just needs a nudge from its users to motivate them to start working out any quirks or at least confirm their code will continue working with the new version.
But yeah, as @Gomez_Adams has stated, 6 to 12 months after a new “major” release is made of most software is a good time frame for waiting for the developers to catch up. Sometimes the changes are massive so it might take a while, and sometimes the new software version might itself change a bit so developers delay their interest in making compatible until things have stabilized.
If it’s been over 6 - 12 months since the major software has been released and the developers of plugins and/or templates still haven’t moved on making necessary adjustments, you’ve nudged them and they haven’t really taken interest… Then it may be time to make the difficult decision to find an alternate plugin/theme to meet your needs.
I typically recommend you keep an eye on plugins/themes developed by third parties to ensure they remain actively developed. A plugin for instance that has had NO activity for more than 6 months may actually be dead and should be removed from your install to prevent issues; security or otherwise.
Peter Knowles | TPN Solutions