Should I keep my 5-year-old electric tank heater as a storage unit if I am about to install a new gas tankless heater?
Your core concern is the amount of time it takes between opening the hot faucet and getting water that is actually hot.  This plan will do nothing for that.
The delay is caused by the volume of water (inventory) that is in the pipe in between the water heater and the faucet.  That pipe is exposed to ambient? Outside? air, and has cooled off. That water must be physically pushed out of the pipe by hot water. It's like a big plunger.
You can figure out the inventory of water in the pipe by taking its interior cross section x its length.  For instance 50' of 1/2&quot; pipe* contains 0.57 gallons of (now cold) water. Going out a 1.5 GPM low flow showerhead, it will take 23 seconds for that to vamonos.
The cure is to make the pipe routing more direct, smallify it, or move the heater closer to the faucet. Each step down in pipe size reduces pipe volume by about half (and thus wait time) - however, too small and that restricts flow too much.
Restricting flow can actually help tanklesses, because a major customer-satisfaction issue with tankless is when the customer draws water faster than the tankless is able to heat it satisfactorily, so they get tepid water - and the customer does not understand the technology, so they refuse to do the one thing that'll fix it -- to reduce flow rate. So they reach impasse and blame it on the tankless.
Note that a tankless-&gt;tank arrangement will only make this problem worse, because the tanked heater will assure the first 40 gallons is OK, then will put a 40-gallon &quot;delay&quot; between action and consequence. So as hard as it is already for customers to connect &quot;reducing flow&quot; to &quot;increasing temperature&quot;, this &quot;40 gallon delay&quot; will be far more confusing.  I would expect bizarre customer-satisfaction issues with this.
Putting the tankless after the tank would mean you can ignore flow rate issues until the tank empties, as the tankless will not operate at all.  Once the tank goes tepid, the tankless will be doing &quot;all the heavy lifting&quot;, and then flow rate will affect heat. Again, very complicated, you shouldn't have to take a training class to understand your water heater.


* 1/2&quot; squared x pi/4 = 0.2 square inches or 0.00137 square feet.... x 50' = 0.06 square feet.... x 8.33 gal/sq.ft. = 0.569 gal.