I'm trying to change the datasource of a join. The reason for that is the following: I read the source out of an given Lyr-File, in which the path is abolute. If I load another project, I want the datasource to be the one I load, and not the one standing in the lyr. That's why I need to change that. (This Lyr thing is not important for the question but it may give you a better understanding. Otherwise the lyr can be seen as a static file which contains sources and tablenames).
For illustration: This is the source I load. (Q:datasource\ele1_testings_fgdb\... [ATTACH=CONFIG]34678[/ATTACH]
This is the source standing in the lyr and used for the join. (Q:\datasources\ele1_testing_mdb\... [ATTACH=CONFIG]34679[/ATTACH]