It depends on the kind of forwarder and the type of configuration. You can set sourcetype in inputs.conf and it would be respected for the life of an event with no problem.
http://docs.splunk.com/Documentation/Splunk/latest/admin/inputsconf
If setting it in an input isn't possible, because maybe you want multiple sourcetypes from the same input, then its another story. Universal and Lightweight forwarders do not do parsing, so if you're using those, you'd put your changes on the Indexer. If your using a heavy forwarder, you can put your changes there as data would be parsed by the time it left the output queue.
http://wiki.splunk.com/Where_do_I_configure_my_Splunk_settings%3F
