If endorsed, the constitutional change initiated by Sarkisian will transform the ex-Soviet country's system of government to one led by a prime minister, not a president.
Under the proposal, there would still be a president in a ceremonial role, elected by parliament and serving for a term of seven years instead of the current five.
The opposition alleges that the initiative is aimed at allowing Sarkisian to maintain his grip on power by shifting jobs after he steps down as president when his second term ends in 2018.
Demanding the leader's resignation, some 5,000 protesters rallied Tuesday in Yerevan against the reform and opposition parties vowed to hold mass protests after polls close Sunday evening.
"If adopted, the amendments will perpetuate the rule of Sarkisian and his Republican Party," said Aram Manukyan, an MP from the opposition Armenian National Congress party.
On Thursday, Sarkisian, 61, refused to repeat his earlier pledges not to run for any government office after his second and last term as president expires.
His ruling Republican Party has defended the reforms, claiming they would empower the opposition.
"If the amendments are accepted, Armenia will have a stronger parliament and a stronger opposition with serious leverage for influencing decision-making," senior Republican Party MP Margarita Esayan told AFP.
The Venice Commission, an advisory body of the Council of Europe rights watchdog composed of independent experts in the field of constitutional law, has said the reform will be a "further important step forward in the transition of Armenia towards democracy."
A shrewd former military officer, Sarkisian has been president of the tiny landlocked Caucasus nation of 2.9 million since winning a vote in 2008 that saw bloody clashes between police and supporters of the defeated opposition candidate in which 10 people died.