check tankless warranty, typically not 10 years and payback often takes
that long... make certain your existing propane tank lines and
regulator can supply the tankless big consumption....

you will save the regular tanks standby losses, what about adding a
water heater blanket to reduce standby losses even more?

It sounds like your current tank is inside your heated space? in the
winter the stanby is no loss.......

so you figure payback at 7.5 years?