Yes, as a sentence opener, they are mostly identical and definitely interchangeable.
As for details and nuances, cursory Googling yielded this:
> ****
>
>
> A:50050
> B:
>
>
> ****
>
>
>
>
>
The gist of which is: would be introducing a new topic that is (however loosely) connected to the conversation so far, while would be for absolute _non-sequuntur_.
However, as it also points out, 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 with its many variants (and their plentiful meanings): , etc.