Depends on the costs of your options and how you weight various aspects of ownership. Changing the installation will depend on what you have to do for venting of the tankless and/or any line size changes.
How much gas do you use each year for water heating? You can use that and the rated efficiency of your water heater in order to determine how much you might save with a higher efficiency system.
You can use the difference of installed cost then divide by the annual energy savings to figure the payback in years.
Of course there is no guarranty that either solution will eliminate the specs.