Installing the tank less units will not justify a rent increase but will increase the property value and the residents will benefit from a lower utility bill. I installed two tank less units at my primary residence (a duplex), because of the rebate and the fact that I plan on living here for the next few years, reducing my utility bill. The life expectancy is also estimated to be 5-10 years longer than traditional units. I probably wouldn't install tank less units at a triplex though, especially if I wasn't going to live there for long.
The existing furnaces will likely run for a few more years if you have them serviced (get a skilled technicians opinion of course). A less expensive option would be to change out the water heaters now and phase out the furnaces (maybe do one per year). Otherwise, if you have the capital change them all out now for the peace of mind.
The best thing would be to get estimates for all the options and go from there (cost of converting to tank less units, cost to replace all units with traditional units, cost to change only the water heaters and service the furnaces, etc).