Claire Williams insists the struggling British team is not giving up on 2019.
But both drivers say the updates are actually smaller than expected.
"There were a few things on the plan but unfortunately, after the incidents in Baku, they sort of shuffled things back a bit," said Russell.
Kubica agrees: "These are tests for the future, and for understanding what needs to be improved on the car."
Russell explained that Barcelona could give Williams a clear idea on what to improve for a bigger update in the future.
Kubica said of that bigger update: "It all depends on how the tests with these parts go and what conclusions we draw from them."
But boss Claire Williams denied that it means the team is effectively giving up on 2019, with a view to regrouping and making a big step for 2020.
"No. As we know from history, our team never gives up, no matter what," she said.
"We are well aware of the complexity of the situation, and how difficult it will be to catch up even to ninth place in the championship, but we regard this season as just as important as any other."
However, Claire admitted that Williams could give up on designing its own gearbox in future.
"We are proud that we are a truly independent constructor, but it was one of the reasons that our car was not ready in time for pre-season testing," she admitted.
"We are doing these analyses now, to see if we keep doing everything ourselves or we can hand over some work to contractors and save time and money."