Since upgrading our subversion server to 1.7 I have tried several times to upgrade the subversion plugin on Jenkins (v1.457) from 1.34 to 1.39. Using the plugin manager it downloads the plugin and asks you to restart. However, when you restart the jenkins.war expands and overwrites the newly downloaded plugin with the 1.34 version that I guess is shipped in the war.
The solution is easy. Use the plugin manager to download the plugin and change the ownership of the subversion.jpi file so that the jenkins user can not overwrite it. Then restart. (This will make future upgrades more difficult as you will have to revert the ownership).