Bug#1127723: ssh key change procedure doesn't work any more

Works for me also on Trixie.

The /dev/tty handling in gpg has bitten some people since about a decade or so.

The corresponding GPG_TTY variable is documented at

https://www.gnupg.org/documentation/manuals/gnupg/Invoking-GPG_002dAGENT.html

and you use it yourself in your initial email.

The above recommends to add

GPG_TTY=$(tty)
export GPG_TTY

to .bashrc.

If you do that, the normal cat ... | gpg --whatever should work for you, too.