Small garden with walled beds

My Father-in-Law has a small walled garden and has recently had a small wall built around the edges of the garden to create flower beds. The issue is that the existing plants are now about a foot below the top of the wall and he does not want to damage the plants by covering them in new topsoil.
Does anyone know how he can protect the root of his existing plants whilst filling up his new walled beds with topsoil?
Any help would be gratefullly received?

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