Thanks for the heads-up. We handle this in our upgrade script already:
if apt-mark showmanual | grep -q '^libicu72$'
then
G_DIETPI-NOTIFY 2 'Migrating from libicu72 to libicu76'
G_AGI libicu76
G_EXEC apt-mark auto libicu72
fi
Though it only works if the old package is not somehow removed during the upgrade. This should not happen, did never during my tests, but I will check back with that user on the Roon forum.