A 25 year old female patient wants to get tooth coloured restoration done on her previously amalgam restored tooth. Old restorative material should be removed if ?
Step 5: Removal of Defective Restorative Material  and/or Soft Dentin
Once the caries lesion has been fully exposed (via the initial preparation), careful pulpal and/or axial extension is accomplished so as to remove defective restorative material and/or soft dentin as indicated. Old restorative material may remain on the pulpal or axial walls after initial tooth preparation. His material should be removed if any of the following conditions are present:

The old material may negatively affect the esthetic result of the new restoration (i.e., old amalgam material left under a new composite restoration).
Radiographic evidence indicates caries lesion development under the old material and clinical evaluation confirms the radiographic interpretation.
The tooth pulp was symptomatic preoperatively.
The dentin along the periphery of the remaining old restorative material is soft, or
Retention of the existing material is compromised and the material is easily dislodged. If none of these conditions is present, it is acceptable to leave the remaining old restorative material to serve as a base, rather than risk unnecessary excavation in close proximity to the pulp, which may result in pulpal irritation or exposure. Removal of remaining old restorative material, when indicated, may be accomplished using sharp rotary instruments and light intermittent pressure with or without water irrigation/cooling. 

Water spray (along with high-volume evacuation) is used when removing old amalgam material to reduce exposure to mercury vapor.
Sturdevant operative dentistry. 7th EDITION, Page No:129
The old material may negatively affect the esthetic result of the new restoration
Radiographic evidence indicates caries lesion development under the old material
The tooth pulp was symptomatic preoperatively
Any of the above