EMEA CEO Byrne announced in the spring that he would be leaving the agency after more than two decades.
APAC and EMEA chair Tim Sutton was confirmed in September as his successor.
In an email to staff yesterday, Sutton confirmed that he would take over Byrne's operational duties on 1 January, but that Byrne would remain with the business in an advisory capacity until the end of March.
He also said that Friend, MD of the London office, would become CEO of Weber Shandwick UK and Ireland. Palmqvist, deputy CEO and COO of the firm's Swedish operation Prime Weber Shandwick, is made EMEA COO.
Byrne told PRWeek: "As an agency leader with a commitment to gender equality, I am particularly delighted that my friend and colleague Rachel is to succeed me as UK CEO after my 17 years in the role. Rachel is an amazing, committed and passionate PR talent and leader. I could not leave the business, the UK's most award-winning for many years, in better hands."
He also had advice for fellow agency chiefs on Twitter.
Delighted my friend and colleague @RachelFriendly has been announced today as my successor as UK CEO @WS_EMEA. The first woman to do so, well overdue & truly well deserved. Every middle-aged white guy running an agency should have a #genderequality #diversity succession strategy pic.twitter.com/jTdMDbJEKX
Sutton said of Palmqvist: "He brings to his new role outstanding commercial acumen, high standards of operational excellence and a track record of client service innovation. As we continue to move our business towards innovative, higher-value client-consulting services, Jonas will have a crucial role in the leadership team."
He also noted Friend's role leading the London business through a "transitional period with a collaborative, client-centric approach", leading to its PRWeek UK Agency of the Year accolade this year.
His email concluded: "As we come towards the end of another year in EMEA, you should all be hugely proud of your achievements this year. We have never been in a stronger reputation position and I wish to congratulate everyone on the huge progress, as well across our overall business performance."