What sort of change remainsto be seen.
The biography that beguiles his supporters is replete with danger signals.
Time and again he has shown contempt for the law.
He has urged people not to pay their electricity bills.
After he lost in 2006 his supporters proclaimed him the "legitimate president"
and blocked Mexico City's main street for weeks.
He has said that the courts should be an instrument of "popular sentiment".
His supporters say he has matured,
and that his record as Mexico City's well-liked mayor from 2000 to 2005 shows that he was always pragmatic.
He has made his peace with NAFTA and no longer talks of reversing the energy reform.
He promises to run a disciplined budget, to respect the independence of the central bank and not to raise taxes.
Some of his ideas, like a nationwide apprenticeship programme, make sense.
But he seems to have little idea how a modern economy or democracy works.
He disparages independent institutions, such as the supreme court.
He talks of making Mexico self-sufficient in food and of building oil refineries, which are unlikely to make business sense.
His ideas are simplistic.
He wants to halve the salaries of senior officials, including the president,
and to subject himself to a recall referendum every two years.
Though personally clean, he has formed alliances with politicians who are anything but.
He denounces Mr Pena's education reform, which offers poor children a chance of a brighter future.
Yes, Mr López Obrador has reinvented himself, but as a bundle of contradictions.
That makes his presidency a risky experiment.
The financial markets might tame a López Obrador government.
But a congressional majority for his party might equally encourage radicalism.
It might go well if, say, he curbs corruption or stands up to America over trade.
More likely, progress will remain elusive.
Mexico cannot stop graft without the institutions Mr López Obrador scorns.
And with protectionists at the helm in its two biggest member-states, NAFTA could well collapse.
That would further poison relations with the United States,
possibly imperilling co-operation overdrugs and immigration.
We worry about Mr López Obrador's presidency,
but wish him luck. If he fails, worse may follow.