Only Romain Grosjean will be trying Haas' major car upgrade in Friday practice.
The American team has an objectively fast car this year, but it is the worst in the field in terms of getting temperature into the tyres in the race.
So for Barcelona, the upgrades are extensive, including new wings, floor, bargeboards and even the mirrors.
"We will have one car with the old specification and the other with the new one," said team boss Gunther Steiner.
"So if we have problems, at least we have something to compare with," he told Ekstra Bladet newspaper.
"If everything goes well, we will change the second car on Friday night. We made the decision consciously so as not to confuse ourselves even more," Gunther said.
Kevin Magnussen will be at the wheel of the 'old' car.
Steiner admitted that the start of the 2019 season has hurt Haas' morale.
"When you have races like we had, you always lose a little confidence," he said.
"I am not promising that everything will be fine here," Steiner added. "We were good in testing, so if something different happens here, we have a problem."
Magnussen plays down any hopes that the Barcelona upgrade will be a silver bullet.
"I cross my fingers that it gives us a push in the right direction," he said.
"But it's not like 15 years ago when it was basically a brand new car and a second and a half faster. Now it is more fine tuning rather than something groundbreaking."