I advised someone else regarding this a few months ago. Basically, to go tankless, you'll need an entirely new set of gas lines if you buy a gas tankless system, which is a huge part of the installation cost if you don't install it on your own. If you don't have a conventional (furnace) venting system for your existing heaters, you'll also need to set up a new venting system or essentially install a chimney, which rockets up costs. If you're planning on an electric tankless heater, everything is much simpler, as you don't need venting and whatnot. However, you'll likely need an electrician, since you'll need to modify your power system to run one. Definitely do your research to make sure the payback period is worth it for you. Friend ultimately decided to stick with another gas water heater, since it would have taken 20 years to break even, which was also the expected lifetime of the tankless heater.
