The regulations do not define this particular change of use as 'material'.
However, if your proposed project involves work which will affect the structure of the building, the means of escape and other fire precautions, or affect the access to and use of buildings, the regulations will regard your work as a 'material alteration' (and therefore 'building work') which must then comply with the regulations.
You should also check with the local fire authority, usually the County Council, to see what on-going fire precautions legislation will apply when the building is in use.