Moving drivers from Edinburgh to Livingston is hardly a major move. It's literally half an hour down the road! I'm pretty sure drivers get moved to depots within a company all the time to cover, so it's not as difficult as people make out on here.
Nobody said it was difficult, it's just not as easy as you make out on here.
First of all, and I do imagine that this is indeed the case, Bright Bus Tours (BBT) drivers would need to have it in their contracts that they can be moved onto service work if the requirement arises - if it isn't there, they can refuse. Also, BBT contracts may only have certain hours of work which limits the redeployment of those staff.
Secondly, as many have pointed out they would need the appropriate route training and dare I say type training on the service vehicles. This all takes time to implement.
Thirdly, First Scotland East is a commercial business and the idea of running essential services for the people's benefit went out of the window when these services were privatised - they need to make money, they need to make a profit and without said profit, the company will fold. If BBT is bringing in considerably more fare revenue than the services cancelled in the evenings then as a business (which needs money to stay afloat mind...), they're naturally going to keep the tours running.
Now, none of this is ideal - but it's just the way it is.