A new sample is not needed just because an old result is negative. It makes sense when the original sample does not match the material that is now going to be removed, or when it is unclear which spot the result applies to.
1. The renovation uncovers a new layer of material
If the original sample was of the surface material and the demolition reveals an old adhesive, filler or a board that looks different, the earlier result does not automatically describe the new material.
2. It is unclear where the sample was taken
A report that only says "bathroom" is hard to match to the right spot if both the floor and a wall in the room were tested. If you cannot be sure how the sample relates to the demolition spot, a new, clearly labelled sample is often the safest way forward.
3. The material changes partway across a surface
The floor or wall of the same room can have repairs from different periods. If the colour, texture or way it was laid changes, it may be a different material that needs its own analysis.
4. The original piece was not representative
A clean tile says nothing about the grout and tile adhesive underneath. Take the new sample from the whole layer structure so that the layers that matter for the test are included.
5. The report applies to another flat or another part of the building
A result from the flat next door or another stairwell can provide background, but on its own it does not show what the material is made of where you are going to demolish. Materials and renovation history can vary within the same building.
A good new sample does not repeat the old one at random. It clears up one specific uncertainty: which material, at which spot, and which work stage the result is meant to guide.