Can I expand or intensify a legal non-conforming use on my Ontario property?

Generally, no, not without separate approval. Legal non-conforming use status protects the use as it actually existed at the time the zoning bylaw changed, at roughly the scale and intensity it was being carried out; it isn't a general licence to grow, intensify, or change the nature of that use over time simply because the original use was protected. Expanding a non-conforming use, such as increasing the footprint of a non-conforming building, adding significant new activity, or intensifying the volume of an existing operation, typically requires its own separate approval, whether through a minor variance, a rezoning, or in some cases isn't permitted at all depending on the municipality's bylaw.

The line between a permitted continuation and a prohibited expansion isn't always obvious, particularly for a use that has grown gradually over years, which is why owners considering any change to an existing non-conforming use, even one that feels like a natural evolution of what they're already doing, should get a clear opinion from the municipality or a lawyer before proceeding, rather than assuming the original protection automatically stretches to cover the change.