They must have some sort of application underway already to convert the employment lands - the city would have known this was coming. A developer cannot just rezone employment lands, they have to apply for a conversion request through a special process that takes years and years to complete.
It is extremely difficult to convert employment lands. It can only be done through what is called a "municipal comprehensive review" (MCR), which is a City's statutory requirement to review it's official plan at least every 5 years. When this process begins - entirely at the city's initiative, a land owner can submit a request for the city to review the land use designation on their lands. Then begins a long study and appeals process. The city is considered to have sole discretion on which lands to release from employment lands and which lands to retain.
If this land is allowed to construct residential - it means the city agrees that it should be such.
It isn't a standard OPA to change the land use designation and start building.
To give an example of the timelines involved, the City began its current MCR in 2013, and lands are only beginning to be released for mixed use development now.