The move was necessitated after incumbent Ahmed requested for leave for a personal visit abroad for three months from December end, Congress sources said.
The party's leadership decided to make alternative arrangements after Ahmed informed them about his leave requirement from December 28 to March 7 for visiting Canada, the sources added.
Since Delhi Assembly elections are expected in January or February, the party leadership announced the replacement soon.
After writing to Congress President Sonia Gandhi in this regard a fortnight back, Ahmed had also met her about his leave request.
Chacko had earlier served as the in-charge of the AICC Screening Committees for these states.