Hi James,
thanks for your question. The grid itself was extended from 12 columns to 48 (see also this commit).
Actually there is code in Kibana that should have updated your dashboard when upgrading Kibana (https://github.com/elastic/kibana/blob/master/src/legacy/core_plugins/kibana/public/dashboard/panel/panel_utils.ts#L74). Do you have any idea why this could potentially not have ran? Did you somehow manually manipulate the saved objects during upgrading?
Cheers,
Tim