I don't really see the point of the title--you replace the water heater completely, it does not mean you have to hook all the water up to it at once. If one room is being remodeled, you just shut off the water to that particular room--no need to have both heaters going at the same time, unless you have money to burn using two heaters when it isn't necessary.

I have heard that with the tankless heaters, there is a longer wait for hot water...but you save money on not heating water all day if it is only needed a few times.

We don't want to shut off the water when the remodel to the room is finished. We just want to switch the water source . For example after finishing the hall bath, can water coming from the water heater in the attic be switched via the remodel process to the new tankless water heater that will not be in the attic? But still will need the old water heater for rooms not yet remodeled and whose plumbing hasn't been upgraded to copper,etc. Can both water heaters be running at same time?

For example keeping the master bath and the kitchen still using the water heater in the attic because plumbing will not be complete yet that routes them to tankless. So we will have both tankless not in the attic and the old water heater in the attic working at the same time but just servicing different rooms. Can we do that?

I didn't specify since I'm not a plumber but I can't help thinking that main issue is that you need gas going to both water heaters and water from the outside being routed into both water heaters. Not sure if this is possible to do.

Then after 6 months or a year when all rooms are remodeled and have their galvanized pipes replaced with copper, etc. and have plumbing routed to new location of tankless, the old water heater in the attic can be shut down because it won't be used.

You are aware now that copper is the harder way to plumb a house? There is flexible tubing made from pex, which only requires joints at either end, rather than at every turn? Alternately, that there are now solderless connections for copper piping which are just as good?

And while I did get your point--my point is, at some original point, your current piping hooks to the water heater you have--hook the new tankless to that and you replace the piping to each room as you get around to it--with shut-off valves, there is no need to lose service to each room for more than the time it takes to run the new pipes. And, galvanized pipes are okay unless inspection has shown leaks or you have a big drop in water flow.

Thanks - think we will do what columbusguy1 indicated and get the tankless installed where we want it and run that back to the piping in the attic. Then room by room, we will re-pipe more efficiently and easily and directly from fixture to tankless using pex. THANKS!