What are the consequences of an an employer deciding to take work from one contractor on site and give it to another?

This would constitutes a breach of contract for which the employer would be liable. This situation was illustrated in Carr v J.A. Berriman Pty Ltd where it was stated:

	The clause is a common and useful clause, the obvious power of which, so far as relevant to the present case, is to enable the architect to direct additions to, or substitutions in, or omissions from, the building as planned, which may turn out, in his opinion, to  be desirable   in the course of the performance of the contract. The words quoted from it would authorise the architect... to direct that particular items of work included in the plans and specifications shall not be carried out. But they do not, in my opinion authorise him to say that particular items so included shall be carried out not by the builder with whom the contract is made but by some other builder or contractor. The words used do not, in their natural meaning extend so far, and a power in the architect to hand over at will any part of the contract to another contractor will be a most unreasonable power, which very clear words would be required to confer.