Group M veteran to join competitor group in October.
Account is estimated to be worth $5 billion.
UK agency to be absorbed after fewer than two years in operation.
Appointments follow a series of leadership changes at the group.
GroupM leadership shake-up to be effective from September this year.
Previously, Seamer worked for GroupM in leadership roles across the APAC region, including a term of more than two years as CEO of MediaCom Australia and New Zealand.
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