Hi Bill,
I have looked at a few homes in your exact situation in Salt Lake. This will really depend on the zoning laws in your city but this is how they work in SLC. If a non-conforming use was established before the zone was created or changed then you can apply for a certification from the city. This cert is what a lot of banks will require to ensure that if the property is damaged beyond repair, by no intentional cause of the owner, that the building can be rebuilt. One house I looked at the city wanted to make sure that there wasn't a gap in residential use for no more than 365 days consecutively. If the city could prove that the home was either not lived in for a year or that it was used for any commercial use then they would deny the certification and traditional residential financing would be a lot harder to get.
Something else to keep in mind is that if I did change the use of the land I would need to bring the standards of the lot up to today's standards. One home I wanted would have needed a sewer line added and it was going to be a 500 ft run to the closest sewer. That was a deal killer for me and I walked.
Again this is my experience in SLC and I can expect some similarities but I make no guarantee to how Denver deals with their zoning.