In the latest version of kmail the combo box to change account/from address is missing. It seems to be just a text field now, and you have to manually type in the from address. If you type in one that matches an account, it does not change things like the signature or GPG signing. Seems like something that developers would be aware of. Either a change on purpose, work in progress, and/or something miss-configured when kmail is built on Gentoo. Reproducible: Always Expected Results: Combo box to change account identity/from address that brings up all that accounts settings, signature, GPG signing, etc.
Never mind, seems it was there, but it was not enabled under View -> Identity. I enabled that and it showed up again. May need to ensure that is selected by default.