The conversion process involves converting your existing deed into a master deed, and then creating individual deeds per unit. Figure around $10k in legal costs. You also need a ratio of parking spaces to rental units, although it doesn't have to be 1:1. You also need a way of getting taxes, water and sewer bill and insurance, to be paid by the condo association, which is yet to be formed. So, you get to figure out the percentage of ownership per unit, and people will pay these costs based on their percentage of ownership. For example, if you have a 6 family building you're converting, each new owner has a 1/6th share of the building. If your taxes are $5,000/year and water and sewer=$2400/yr and insurance is $4400/year, your overall fixed expenses=$9,800/year, so as part of the condo dues, you figure 1/6th of $9,800 is the minimum, plus cost of snow removal and building maintenence. Make sure you separate the boilers for common areas vs per unit by hiring a plumber if you haven't done so already. In other words, the building is now longer owned by you, the landlord, but is being sold into individual deeds, so the owners of the units need to manage those expenses combined that you used to manage yourself.
Another thing to keep in mind is the "right of first refusal". That means, your tenants have the first chance to buy the units, before they are offered to the general public. So, if you are collecting rental income, you need to file with sufficient notice to your tenants, of your intentions of condo conversion. If you are going to be spend money on converting each apartment from "rental quality" to "condo" quality, it helps if you can prequalify you tenants for what will be a mortgage payment rather than a rent payment. Likely, if your tenants can afford the unit once it's improved and updated, if you work with them to improve their credit and ability to purchase the unit. On the other hand, if you have a mass exodus from the building, be sure you can carry the note completely on your own, without rental income to offset your carrying costs while you do work on the apartments and then subsequently sell them. Good luck and I hope this advise helped.