When you are through changing, you are through.

Bruce Barton (1886-1967) American author, advertising executive, politician

Article Title, The American Magazine (1929?)
    (Source)

Barton was a regular contributor to The American Magazine. Both the cited source (from 1929) and this suggest this was an article he contributed no later than 1929.

The saying has been misattributed to a number of more recent consultants, motivational speakers, etc.