you will need large natural gas line, and perhaps a new meter too.
there will be a delay between water on and water hot, time lag for
ignition and heating. you wouldnt save much energy, standby losses are
actually pretty low and help to heat the home if heater is in
flue may not be big enough most water heaters 35K BTU, tankless over a
100K BTU... Tankless must do all the work heating a standard tank
does, on a flash basis in a tankless.
Check for local affordable service, newer tankless are high tech and
may need qualified service tech, and perodic cleaning heat exchanger
with acid, espically in hard water areas
heres a good experiment turn your current heater to vacation or off,
go out for day. you will find temp doesnt change much...
low flow situations may be below trip level for burner on, so no hot
on many tankless and all power vent tankless no electric means no heat
at all. power is necessary for unit to function