When was the Britannic turned into a hospital ship

I know she had her maiden voyage in December of 1915. But when was she made into a hospital ship? I saw a resource that she was orignaly going to be used for troop transport, Like her longer lived sister, Olympic. If so, did she make her maiden voyage as a hospital ship? Or was it for troop transport? If so, when was she made into a hospital ship?
 
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