Mercedes, who provide McLaren's engines, have won all three races so far this season, but the last race in Japan was closer between the two teams.
Oscar Piastri ended up second to Mercedes' Italian Kimi Antonelli at Suzuka and might have won but for a safety car period helping his rival.
"There was always the idea to deliver a completely new car, especially from an aerodynamic upgrades point of view, for the North American races," Stella told reporters at the team's UK factory in Woking.
"We could keep up with this plan. Obviously, the fact that the calendar has been changed sort of helped a little bit, like I'm sure it helped all the other teams that could work more streamlined towards upgrading the car rather than being busy with racing."
There has been no racing in April because the Bahrain and Saudi Arabian Grands Prix were called off due to the Iran war and its impact on the Gulf region.
That means Miami will be the fourth race of the season rather than the sixth.
"Across Miami and Canada we will see an entirely new MCL40 ... this is what I would expect of most of our competitors so it's not necessarily going to be a shift in the pecking order, it will be effectively just a check who has been able to add more performance within the same timeframe," said Stella.
The team are third in the standings, 89 points behind Mercedes who finished the first two races one-two.
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