You can install a new crontab reading it from a file or from standard input. To modify an existing crontab you can retrieve it using option -l, then apply your changes and re-install it again. For example:
crontab -l | sed 's%/etc/service/start -c%/etc/service/start%' | crontab -
you have only to think about the editing command (sed) to apply the changes you have in mind.