Is oil change mandatory when changing solenoid valves?

about 10,000 km ago I changed the gearbox oil. now I want to replace solenoid valves. Do I have to change the gearbox oil again during this change, or is it OK to use the existing oil? what?

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I don't know what is mandatory, but they had changed mine, so change it if it's not mandatory

Despite my intense insistence, the master did not change the oil while my vehicle's solenoid valves were changing. The condition of the oil is good. For now, he did not need to change, he said 15.000 km later, let's see again. so they keep the old oil in a clean container. then they use it again. & nbsp;