I use Dawn myself and have run into the same problem. Your existing customized theme remains available after the update, but that doesn’t transfer its customizations into the new Dawn version.
Where possible, it helps to keep custom code in separate snippets or asset files. However, changes made directly to existing Dawn files still need to be checked with every update. Simply copying the old modified files into the new version isn’t safe, because that could overwrite changes Shopify introduced in the updated theme.
This exact issue with Dawn led me to create a paid Theme Update Audit. It compares your customized theme ZIP with the updated version and identifies which changes are present, missing, partial, or conflicting. In your case, this would include changes such as the WhatsApp icon and the additional collapsible-row behavior.
The audit includes guidance on what needs to be addressed. More details are available in this Ask & Offer post: Theme Update Audit for Custom Code