The requirement to upgrade is not well defined in codes. Here are the two main points in changing a light fixture. Wire temp rating required for new fixture and requirement for bonding. If a fixture replacement requires bonding as would for a 2 wire receptacle on K & T wiring or the new fixture requires a higher temp rated wire than is available in the outlet box then an upgrade is required. Even an antique fixture that is rewired will be rewired with a never before required bonding conductor.
If requested I have approved gfci protection where bonding a new fixture is impractical.