The Ontario Planning Act governs how Zoning By-laws can be used for regulating the use and development of land. It also provides for what is known as non-conforming rights (also more generally know as "grandfathering").
What this means is: if you as a property owner have a use on your land which is legally established, that met the Zoning By-law regulations that were in effect at the time the use was established and then a new Zoning By-law is passed which has the effect of no longer permitting the use on your property, then you will have non-conforming rights to continue the use on your property. If the use is eventually discontinued, you would lose the non-conforming rights and will not be able to re-establish the use which is no longer permitted.
Similarly, if a new Zoning By-law is passed that changes the regulations, and the development on your property does not meet the regulations (for example such as height or setbacks), this means that your building is considered to be non-complying.