Hello, the Studio ZIP only contains what Studio registered itself (fields, views, actions created through Studio). Reports, approval rules or fields on non-standard models can be missing, and no single export lists everything, which is why you end up checking view by view.

For an upgrade preparation there is a more reliable way to know what changed: compare the two databases directly. That is what I built Dooplik (dooplik.com) for. I am an Odoo partner working on verticals where the same customizations must be reproduced across databases and versions. You connect two instances (external API, nothing to install in Odoo), for example your production copy and a test database, and Dooplik compares them object by object (fields, views, automations, actions...). You see exactly what exists on one side and not on the other, you select what to propagate, and you get a per-object audit report (Excel, PDF, JSON). Missing modules are installed first and relations are kept through XML IDs, including across different Odoo versions.

Comparing two instances is free and unlimited (free account, no credit card). To be sure nothing is forgotten and to save a lot of time before an upgrade: dooplik.com