The move is part of the team’s attempt to get to the bottom of its recent struggles, with Grosjean having felt uncomfortable since a major upgrade package was introduced at the fifth race in Spain.
The Frenchman will retain the original spec for the whole weekend at Silverstone, while teammate Kevin Magnussen will retain the latest iteration, allowing the team to get three days of back-to-back data.
Among the differences between the two cars are the floor, nose, front wing, rear endplates and turning vanes.
“When we introduced the update I wasn’t fully convinced about it in Barcelona,” said Grosjean when asked by Motorsport.com.
“But the data suggested it was good, so we put it on, and we kept going. But since a few races I really felt better with the initial package so I wanted to come back in terms of feeling. And obviously the last two races in terms of performance have proven to us that we need to understand a bit more what’s happening.
“Early in the year, we had very good quali, and then the race pace was not as good. But I think we understood what happened in China and also in Bahrain, and what we could’ve done better.
“So that was positive – I think our pace there had more potential, while recently the race pace does not have much potential.
“I think I really extracted the best of it out of the last three races, and Kevin has got the upper hand in quali but in the race completely the opposite, I almost lapped him twice – but it’s P16.”
Grosjean said team boss Gunther Steiner gave the go-ahead for the move back to the original spec to create an opportunity for the team to work out where it might have gone wrong in recent weeks.
“Gunther has spoken, after the race in Austria, and he says, ‘Look, we need to try something, if we keep going the same, it’s the same thing.’ I said, ‘Look, my feeling is not good with that package, I felt better in the other one, can we go back and see what it’s like?.’
“Obviously it’s a hard call, and it’s very difficult for all the engineers involved in the development to come back. But on the other hand maybe there are things that we don’t measure that my ass can feel, but the sensor don’t see.
“So, that’s really why we’re coming back here, and see if with that package we’ve got more options to set up the car to go faster in the race. It was lucky they went behind the shelf in Banbury and they could find an old floor and an old everything.”
Grosjean is not concerned about trying the old package at a track where it has not previously been run.
“At least we’ve got two cars running two different packages, as much information as we can in terms of sensors and then it’s just going to be pure pace, and see.
“But as I said, the new package, I never really felt comfortable with it, and I didn’t have that problem with the old one. So, if that’s any correlation in the fact that we cannot go as fast as we want, that’s what we want to find out.
“It’s decided for the weekend. The change is too big, it involves some chassis-moulding, so we’ve got to run it for the weekend.”