In most houses a single tank sized for the biggest tub-fill is enough, and for your family of four that's probably going to be fine too. Depends on how big that soaker-tub really is. I suspect you could do just fine keeping the existing 50 gallon unit as long as it can fill the soaker tub. The only time you'd need more is if all four of you are showering at the same time, taking full advantage of the 6 baths. Keep it, add the (code required) tempering valve, and crank up the storage temp if it doesn't quite fill the tubbie at 140F or whatever.
If you find you are running short of showering capacity with the existing unit, you can probably still keep it. Installing a drainwater heat recovery heat exchanger rated at 50% or better return efficiency would provide more total showering time than a 50 gallon tank with a high storage temp and a 75K burner, and would cost less to install. For showering loads it's like adding to the burner size, but uses no fuel. EFI is probably still cheapest place to get them in the US. (Home Depot has them online, at a full retail kind of price, as does the manufacturer's buy-direct page.) Comparative efficiency rating lists from third-party testing of different models are maintained by Natural Resources Canada to be able to compare apples-to-apples. At lower flow the efficiency is a bit higher, at higher flow a bit lower, but the BTU-rate increases. (40% energy return on a 10gpm flow is a lot more BTU/hr than a 55% return on a 2.5gpm flow.)
But it's only good for increased showering capacity, since the drain has to flow at the same time as the hot water to get the exchange. They do nothing for tub filling capacity.
When it's time to replace the 50 gallon unit, you'll still have options. The 50 gallon condensing Vertex has a 76KBTU/hr ~95% efficiency burner and puts out considerably more heat than a 75K burner on a typical tank (~80% steady state combustion efficiency), and you wouldn't even have to crank up the storage temp. But if the house is heated with a boiler, an "indirect" tank running as a zone off the boiler is often the best overall solution.
Seem like a reasonably staged "see how it goes" plan? That way the expense is deferred until you KNOW you need an upgrade, and you're not scrapping equipment and replacing it with something else that may or may not cover your actual loads. And adding the drainwater heat exchanger buys your more showering capacity for less money than a bigger tank or bigger burner, for roughly the same installed cost of a plain-Jane 50 gallon hot water heater.
FWIW: A 76K condensing burner like the Vertex is enough to supply a continuous 24/365 two gpm shower with margin to spare at D.C.'s incoming water temps, but a 40K/80% burner in conjunction with a drainwater heat exchanger (which uses less fuel) would keep up too.