Hi @Jonathan_Wynne ,
However, in the Web version I cannon find an option for doing so?
As far as I know, this is currently not available in Outlook web browser.
Given this, I'd recommend change the setting to make From line always display so that it can be a bit easier to change the from email address when composing the reply.
If you are using Exchange online account, you can find the setting via Options > Mail > Compose and reply:
If your account is hosted in on-prem Exchange like Exchange 2016 or Exchange 2019, you can find the setting via Options > Mail > Layout:
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