The Victoria Line is closed between Seven Sisters and Walthamstow Central, and services will not be as frequent between Seven Sisters and Brixton.
Transport for London (TfL) said the work would mean an extra 12 trains per hour running on the line.
The line carries more than 200 million passengers a year.
TfL said at present 24 trains per hour were able to travel beyond Seven Sisters, but the work will mean 36 trains can pass through.
London Overground Services are expected to be full on departure from Walthamstow Central, and so will not stop at Clapton, while St James Street will be exit only.
TfL said the three-week closure meant the rest of the Victoria Line, the Central Line between Bond Street and Leyton and bus services in north-east London were expected to be exceptionally busy.
Rail replacement buses will run from Walthamstow Central to Seven Sisters as well as from Walthamstow Central to Stratford and Stratford International.
Thousands of London commuters are facing three weeks of travel chaos after a large section of the Victoria Line was closed for refurbishment.