You can also use the REST V2 API:
http://fmeserver/fmerest/v2/apidoc/#!/schedules/enabled_put_5
Hi @sunsilk11,
As itay mentioned only a few parameters are optional. Instead what you could do is first use the GET response to read in the current schedule set up, then use a StringReplacer to change the "enabled":false to "enabled":true and then use a second HTTPCaller with the PUT request to upload the modified response.
Looking at the documentation shows that only a few parameters are not optional, these are the only ones you need to supply, I never tried it my self but I think you cannot just change only the enable tag.