How to use Baseline Update API?

I am using the Baseline Update function from a web service call.

The goal is to change the version for an object already included in a publication's baseline.

My application's request is received and processed by Tridion Docs without returning any errors.

But the publication baseline remains unchanged.

Here is the XML I am sending with the request:

<modifications>

<object ref="[objectGUID]" action="update" versionnumber="[version_number_I_want_pub_to_use]" source="Manual"/>

</modifications>

Am I missing something?