The Blues have replaced head coach Tana Umaga with former New Zealand fly-half Leon MacDonald following an end-of-season review.
MacDonald was appointed as an assistant to Umaga in May - with his deal set to begin at the beginning of next season - but the pair have now swapped roles.
Umaga, a former All Blacks captain, will now be responsible for the defence in his new role as an assistant.
He said: "After the end of season review I signalled that my passion lies with my new responsibility as defence coach and I believe that is where I can make the greatest contribution and difference at the Blues.
"I support the decision to appoint Leon and I believe that the coaching team of Leon, Tom (Coventry) and myself is the best that we could have at the Blues for the 2019 season and beyond."
MacDonald's role at the Blues will be his first as a head coach in Super Rugby, although he has coached Tasman in New Zealand's provincial championship.
The 40-year-old was an assistant at the Crusaders, but quit last year to spend more time with his family.
MacDonald and Umaga will be joined on the coaching team by Tom Coventry, who was appointed in May as forwards coach for next season.
The Blues' coaching shake-up comes after a disappointing season in which they finished second bottom in Super Rugby, with just four wins all season.
Umaga admitted: "It's been a challenging three years for me working for the first time as a Super Rugby head coach. I have learned a lot and I know I still have a lot more to offer.
"I will be 100 per cent behind helping to implement some of the strategies that we have identified that are still works-in-progress."
The Blues begin their 2019 season against defending champions - and MacDonald's former side - the Crusaders on February 16.