New Orleans, Louisiana (CNN) -- As oil again again flows freely, crews are working around the clock to replace a containment cap on the ruptured underwater well in the Gulf of Mexico, BP said Saturday.

The company hopes to install a better-fitting one in the coming days, BP senior Vice President Kent Wells said.

"We're on plan," he said hours after the old cap was removed.

Live video showed robots in the process of removing six bolts from the apparatus so that the new cap can be positioned. The bolts may all be removed by Sunday.

The old cap had been diverting about 15,000 barrels a day (630,000 gallons) to a ship. BP still is recovering an additional 8,000 to 9,000 barrels a day (336,000 to 378,000 gallons) through a line connected to another vessel, the Q4000.

Wells said the sealing cap operation is expected to take four to seven days, with favorable weather helping the process along. In upcoming weeks, there will be an increase in the current oil containment work. A fleet of skimmers will help recover the oil now making its way to the surface.

"As we start to ramp up the additional containment capacity, we should see less and less flow," Wells said.

Wells said crews are in the final stages of hooking up the Helix Producer, another oil recovery vessel, to the well. The vessel is expected to begin collecting oil by Sunday and officials hope to reach full collection capacity of 20,000 to 25,000 barrels (840,000 to 1.5 million gallons) per day within three days.

"We're in the final stages in doing the pressure testing" on the Helix Producer, Wells said.

If successful, the effects of the containment cap operation could halt the oil gusher that started April 20 after a deadly explosion on the Deepwater Horizon rig in the Gulf of Mexico.

Officials have said such a fix would be temporary, and the permanent solution would still be completion of a relief well. There are two relief wells under construction, with one expected to be completed in August. One could intercept the leaking well as early as the end of July.

Over the next two to three weeks, 60,000 to 80,000 barrels (2.52 million to 3.36 million gallons) a day should be contained, Wells said. Scientists estimate that 35,000 to 60,000 barrels of oil are spewing daily from BP's breached Macondo well.

The company also says the sealing cap work "is intended to run in parallel with the installation and start-up of the Helix Producer."

BP says there will be a period of decreased oil and gas capture from the wellhead during the cap replacement. It said another recovery vessel, the Q4000, "should continue to capture and flare oil and gas." There will be other recovery vessels and skimmers deployed.

"This can be done very effectively," Wells said.

Wells also said there will be "significant measurement capability" added to the new cap so officials can get a good idea of the flow rate.

BP said in a statement that the new cap "should improve containment efficiency during hurricane season by allowing shorter disconnect and reconnect times."

Wells echoed much of what retired Coast Guard Adm. Thad Allen said Friday.

Allen said he approved the cap-switch plan in order to take advantage of favorable weather predicted for coming days and because, once the switch is complete, the resulting capacity to contain oil "will be far greater than the capabilities we have achieved using current systems."

Allen stressed that once the capping device is on, "we would get the most accurate flow rate to date," he said.

Meanwhile, Allen on Saturday announced that Rear Adm. Paul Zukunft is scheduled to relieve Rear Adm. James Watson as the federal on-scene coordinator Monday.

Zukunft, who has been in the region for several weeks overseeing strategic planning, is the Coast Guard's assistant commandant for marine safety, security and stewardship. Watson will return to his previous duties.

CNN's Sanjay Gupta, Vivian Kuo and Aaron Cooper contributed to this report