The short answer is: yes, but there are quite a few snags to carrying out such a tire change. The sports regulations of Formula 1 do not necessarily state that it is prohibited to change less than four tires during a designated pit stop.
Article 24 paragraph 2 of the regulations states that ‘each set of tires must consist of two front and two rear tires, all of which must be of the same specification’. This means that a driver is not allowed to drive on, for example, two medium front tires and two hard rear tires. In principle, a rider has to do the trick on four tires of the same compound.
In the sentence after that, however, is the catch, which Mercedes and Bottas have used. The tires must in principle be of the same compound, but ‘sets of the same specification may be mixed, after qualifying’. This means that during the race, the teams are not obliged to change all tires during a pit stop. Yet in practice it is a logical choice, because a new tire is always faster and lasts longer than if you let the tire sit.
But why didn’t Bottas change his right front tire during his last pit stop? That had everything to do with the cause of the pit stop. During the safety car phase, caused by Lance Stroll being tapped by Daniil Kvyat, Mercedes saw that Bottas had a slowly deflating tire caused by being driven through debris. Mercedes then decided to send Bottas back out on the hard tire. As a result, Bottas only had a set of medium tires left. When Bottas then came in again on lap 25 for medium tires, the old set of tires had to be put under when he made his last pit stop on lap 38. However, because there was no right front tire left – it had become punctured – the right front tire had to remain in place. So this was thirteen laps older than the other rubber under the Mercedes.
With nineteen laps to go, it was impossible for Mercedes to put a tire from another compound under Bottas’s car. Drivers are in principle also allowed to drive with tires of different compounds, but only for a short time. According to article 24 paragraph 4, ‘any driver who uses tires of different specifications during the race may not ride them for longer than three laps’. Otherwise, the driver will face a penalty, the regulations prescribe. However, unlike in the question above, there is no demonstrable reason for this. After all, a second pit stop would take so much time and driving on different tire compounds would not be particularly advantageous than driving on the same tires. Still, the damage for Bottas would not have been very great if he had done that: the Finn eventually finished eighth, scoring only four points.
View Bottas’ pit stop below, where his mechanics still pretend to change the tire:
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