git filter-branch. That of course modifies the repository and changes the hashes that are up to now linked to the SVN-revisions. Therefore in a second step these linked hashes need to be replaced in the revmap file of git-svn located in
.git/svn/refs/remotes/origin/trunk/.rev_map.[UUID of the SVN-remote from .git/svn/metadata].
greping for old hashes and replacing them in the found files then did the trick. So now we can do a test run on a box that we can set up a cronjob on that can then push the modified repo to the php git server.