what's the difference between ところで and ちなみに ?
Are they always/often/seldom interchangeable?
Yes, as a sentence opener, they are mostly identical and definitely interchangeable.
As for details and nuances, cursory Googling yielded this:
The gist of which is: ちなみに would be introducing a new topic that is (however loosely) connected to the conversation so far, while ところで would be for absolute non-sequuntur.
However, as it also points out, ところで will often introduce a "new" subject that came to the speaker's mind because of the previous subject (and is therefore not all that disconnected).
Bottom line, they are pretty much interchangeable (but the nuance exists).
Note: make sure not to confuse ところで with its many variants (and their plentiful meanings): ところです, ところが etc.