Romain Grosjean could return to the old-spec VF-19 after the Italian Grand Prix next weekend.
The Frenchman has been running the Australia-spec car since the British Grand Prix, finding more comfort with it as Haas investigates its lack of race pace.
Grosjean's teammate Kevin Magnussen has been sticking with the up-to-date package, however Grosjean will run the same spec car at Spa-Francorchamps.
The 33-year-old confirmed that Haas didn't have a choice to revert to the new package, as the old-spec car is far from being suited to the low downforce circuits of Spa and Monza.
"It's not a decision, it's a no-choice option," Grosjean said. "The old downforce package is not runnable on the old car. I knew for a long time, since Silverstone, that I would be running the new car here and Monza.
"We didn't have a choice, the wings don't fit on the old car," Grosjean added.
However, Grosjean could find himself back behind the wheel of the old-spec VF-19 in Singapore, while he says that using the old car in Russia is "more than likely".
"Then we can decide for the next few races, it doesn't mean that the old lady is not going to come back," Grosjean said. "Maybe Singapore, let's see how that one behaves, Russia more than likely."
Grosjean ended the second practice session from Spa in 12th place on Friday, three-tenths ahead of teammate Magnussen, who found himself in 18th.
Bahrain International Circuit - Winter testing