I\'ve been working on code checked out from the development line and discovered that the changes made might be breaking changes and need to be moved to an experimental branch be
You should create a branch from a known sourceURL (this would be your 'development line' you mentioned in the question) first:
sourceURL
svn copy sourceURL branchURL
Then, switch to that branch:
svn switch branchURL
And commit your changes:
svn commit