https://the-race.com/formula-1/lessons- ... 1-testing/
this article on the-race.com gives some indications of a pattern that mercedes has followed whenever there are significant rule changes. it quotes a lot of james allison's interview statements at the end of 2019 season. he says, what they did in 2019 testing, is something they did in 2017 also.
Mercedes opted to run in the first test in Barcelona with a version of the car that was signed off very early – in November 2018. The aero package could then be manufactured and used for the first four-day test in Barcelona.
Technical director James Allison explained the thinking behind this at the end of that season.
“We did the same thing in 2017,” said Allison. “There was a reasonable aero reg change in 2017 and we built two cars then. It was exactly the same rationale, knowing that at the beginning of big changes the slope is so steep – a week finds you a tenth, two weeks 0.2s – that could be such a big difference when you come to line up on the grid that you want to make sure that you build your final set of clothes as close to the death as possible.
if that approach to tackle regulation changes has continued, it's fair to expect some upgrades on the car that would race in bahrain.