Apparently the attitude changed when the plan didn't pan out and Shokin was found to be not following it;
"But Ukrainian prosecutors and anti-corruption activists with knowledge of the matter argue that the timeline of developments in the Burisma case and Shokin's stint as chief prosecutor simply does not fit the narrative being put forward by Trump and his allies.
Moreover, they say that Shokin himself was the biggest obstacle standing in the way of the investigation.
U.S. President Donald Trump asserts that Joe Biden pushed for the ouster of Ukraine’s chief prosecutor to quash a probe into a Burisma, a gas company with Biden's son on its board. Officials and anti-corruption activists in Kyiv say Prosecutor-General Viktor Shokin had shelved the case long...
www.rferl.org
'Burisma Case'
Daria Kaleniuk, executive director of the Kyiv-based Anti-Corruption Action Center (AntAC), told RFE/RL that Shokin "dumped important criminal investigations on corruption associated with [former President Viktor] Yanukovych, including the Burisma case."
Biden did demand that Shokin be removed. At an
event at the Council on Foreign Relations in New York in 2018, Biden seemed to boast about it, saying that during a visit to Kyiv -- likely in December 2015 -- he told Ukrainian officials: "We're leaving in six hours. If the prosecutor's not fired, you're not getting the money."
"Well, son of a bitch," Biden continued. "He got fired."
Shokin was indeed fired, but not until March that year.