We typically try to advise our client to stay with roofing of comparable
weights and perhaps add new 1/2" plywood sheathing, (over the existing spaced
sheathing) to improve diaphragm rigidity.
When they insist on heavier roofing, we'll insist on checking both vertical
and lateral load capacities for the existing building. Usually it would mean
that the scope of work for the building will be beyond merely re-roofing. It
would probably involve structural upgrade for added seismic and vertical
loads. I would use the same Code the building was design for unless the
client requests more stringent design.(fat chance) The rationale is that the
building would do no worse than as if it was designed for the heavier loads
at the time.