How to convert elastic beanstalk classic load balancer to application load balancer on a running application?

It is not possible to set a a load balancer type except at creation time. You can use elastic beanstalk cli and aws cli to clone the application with the same config and version. To get the deployed application version run:

aws elasticbeanstalk describe-environments --application-name ${APPLICATION_NAME} --environment-names ${SRC_ENV_NAME} | jq -r '.Environments | .[] |  .VersionLabel'

The jq pipe filters out the rest of the json blob.

After that, you can save the config of the curent appication using:

eb config save $SRC_ENV_NAME --cfg "${SRC_ENV_NAME}_save"

Then create an application clone using:

eb create $NEW_ENV_NAME --elb-type application --cfg "${SRC_ENV_NAME}_save" --version $APP_VERSION

Where APP_VERSION is the string extracted in step one.