If other packages have requirements on the package you are changing, your development branch may fail to meet those requirements (which will result in "Your requirements could not be resolved to an installable set of packages."). To fix this, you can do an inline alias to that all other packages will see it as a specific version.
If you started with:
change it to:
"massivescale/celery-php": "dev-php-amqplib as 1.2.3"
(Thanks to Rafael Dohms for the tip.)