If Powell’s theory is correct, the tomb in which Edmund of Langley and Isabelle of Castile are buried was intended originally for Richard II and was reallocated after Anne of Bohemia died and Richard decided to commission the well-known joint tomb at Westminster.
I am unable to explain why King Richard ‘commanded’ his aunt to be buried at King’s Langley. She died in 1392, and he cannot yet have decided to have his new Westminster tomb built. It may be that he wanted her close to his own intended burial.
Given that Edmund of Langley was not exactly over-provided with cash, it may be that he was only too glad to accept a second-hand (but high quality) tomb for his wife, and ultimately for himself. Only very modest modifications, such as adding their heraldry, were necessary. We may assume that it was simply moved over the locus where Isabelle lay, with minimum fuss.