How to pass boolean parameter value in pipeline to downstream jobs?

In addition to Jesse Glick answer, if you want to pass string parameter then use:

build job: 'your-job-name', 
    parameters: [
        string(name: 'passed_build_number_param', value: String.valueOf(BUILD_NUMBER)),
        string(name: 'complex_param', value: 'prefix-' + String.valueOf(BUILD_NUMBER))
    ]

Assuming

value: update_composer

was the issue, try

value: Boolean.valueOf(update_composer)

is there a less cumbersome way in which I can just pass ALL the pipeline parameters to the downstream job

Not that I know of, at least not without using Jenkins API calls and disabling the Groovy sandbox.

like Jesse Jesse Glick and abguy said you can enumerate string into Boolean type:

Boolean.valueOf(string_variable)

or the opposite Boolean into string:

String.valueOf(boolean_variable)

in my case I had to to downstream Boolean parameter to another job. So for this you will need the use the class BooleanParameterValue :

build job: 'downstream_job_name', parameters:
[
[$class: 'BooleanParameterValue', name: 'parameter_name', value: false],
], wait: true