I solved this but posting it in case someone else finds it useful.
Aim: have an input semi-linked to a dropdown. When the dropdown changes, change the input value. But as soon as you change the input manually, lose the link to the dropdown value.
Result: this is really easy to do which I found surprising. Just set the “initial” value of the input to follow the dropdown value.
So the “Initial” value of an input is not just the initial value - it is the default value unless and until you change the input yourself manually.
Demo: https://moraytestzone.bubbleapps.io/version-test/change_input?debug_mode=true
Editor: https://bubble.io/page?type=page&name=change_input&id=moraytestzone&tab=tabs-1
Demo: