Topic Shift (By The Way)

Changing the subject in conversation.

ところで vs ～ついでに: Use ところで. It is the plain topic-change marker and needs no connection to what came before. ついでに is not a topic shift at all: it adds a second errand onto the first ('while I'm at it'), so it needs an action to piggyback on.

Something just said reminded you of a related topic.

A。そういえば B。 vs A。さて B。: Use そういえば. It marks the new topic as prompted by what was said, which is why it sounds natural and unforced. さて is a deliberate move to the next item, the connective of agendas and speeches, and it ignores whatever triggered the thought.

Adding a side note that supplements the topic rather than replacing it.

A。ちなみに B。 vs A それはそうと B。: Use ちなみに. It appends related information and stays on topic, which is why it works in writing too. それはそうと drops the current topic to raise a different one, so it reads as an interruption.