replace string in ros2 launch command xacro

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Hi all, i need to replace a string in an urdf after the xacro command is executed. In the launcher, I load the xacro like this:

    robot_description_content_1 = Command(
    [
        PathJoinSubstitution([FindExecutable(name="xacro")]),
        " ",
        PathJoinSubstitution([FindPackageShare(description_package), "urdf", description_file]),
        " ","safety_limits:=",safety_limits,
        " ","safety_pos_margin:=",safety_pos_margin,
        " ","safety_k_position:=",safety_k_position,
        " ","name:=","ur1",
        " ","ur_type:=",ur_type,
        " ","prefix:=","sweepee_1/",
        " ","prefix_rc:=","sweepee_1",
        " ","simulation_controllers:=",initial_joint_controllers_1,
        " ","use_fake_hardware:=",use_fake_hardware,
        " ","sim_gazebo:=",sim_gazebo,
        " ","sim_ignition:=",sim_ignition,
    ]
)
robot_description_1  = {"robot_description": robot_description_content_1}

After loading it , I need to replace a keyword with another.
The problem is that the command is not yet a string and I don't know how to act on it. I would use the xacro package directly but I have a lot of PathJoinSubstitution, and I cant deal with them in the xacro.
Is there a way to do this ?
Thanks