can I change arg value in roslaunch?

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Hi! Is it possible in roslaunch to change the value of an arg? I would like to write a launch file for rosbag record with arguments about which topics to record. Like this:

roslaunch record record.launch record_scan:=true record_image:=true

where the launch file might look like this:

<launch>

    <arg name="record_scan" default="false" />
    <arg name="record_image" default="false" />

    <arg name="scan_topic" default="" />
    <arg name="image_topic" value="" />

    <group if="$(arg record_scan)">
        <arg name="scan_topic" value="/scan />
    </group>

    <group if="$(arg record_image)">
        <arg name="image_topic" value="/image />
    </group>

    <node pkg="rosbag" type="record" name="rosbag" output="screen" args="/tf /odom $(arg image_topic) $(arg scan_topic)"/>

<launch>

If I try it like this, ros complains:

Invalid <arg> tag: arg 'scan_topic has already been declared.

Is there some way to do this? I also tried nested group statements and putting an unless statement instead of declaring the arg before, but both didn't work. Thank you for your help!!