changing the subject; on a different note; by the way
A casual conversational phrase used to smoothly shift topics, similar to 'by the way' or 'speaking of which'. Often used in spoken Japanese or informal writing.
話変わるけど、来週の予定はどう？
Changing the subject, what's your schedule like next week?
話変わるけど、あの映画もう見た？
On a different note, have you seen that movie yet?
A fixed phrase from 話 (はなし, 'talk, topic') + 変わる (かわる, 'to change') + けど ('but, though'), literally 'the topic changes, but...'. The exact origin as a set expression is uncertain, but it is a natural conversational pattern.