Excerpt below from the 2001 regs. It would appear to me that if the current use as insurance sales is covered by the description ”financial services” or ”professional services(other than health or medical services) that your development would be ”exempted development” as both would be class 2 uses. If it falls under Class 3 it would be a change of use.
CLASS 2
Use for the provision of—
(a) financial services,
(b) professional services (other than health or medical services),
(c) any other services (including use as a betting office),
where the services are provided principally to visiting members of the public.
CLASS 3
Use as an office, other than a use to which class 2 of this Part of this Schedule
applies.