Yes, the numbers/symbols can be substituted to give the same outcome as Einstein's formula, but you miss my point in doing so. Perhaps if I create a thought experiment that will illustrate the “point” that I'm trying to convay.
Lets say that Newton has an apple and he throws it agains a wall and says that the harder he trows it the more energy it will contain as it is flying, Einstein comes along and says, that you are right but looking at the situation a little bit deeper, it turns out that the apple contains energy even when it is not flying anywhere, then I come along and say, you both are right but if we look at the situation even closer, then we will see that energy is always kinetic in nature and if something is moving in the stationary apple, then it requires that there must be space where that something is moving, and since Einstein is right in saying that no matter how small of a piece of the apple we take, say an electron, and stop it from moving in relation to anything in the universe, it still will have kinetic energy with in its structure. Now the movement must happen in space, so we can not separate space from the formula of intrinsic energy of mass, the assumption is that space in its fundamental state is empty volume, if that is so, then there is something in that space that moves and fills that volume, today that space filling “stuff” has been given the name Higgs field. The assumption then is that this Higgs field condences or compresses to form the various, internally moving, mass particles that we are familiar with, such as protons, neturons, electrons, etc., and since this Higgs field can move, perhaps it also forms the photons that also move. Suppose that this speculation is true, then all that is needed in the universe is the basic incredients of volume, Higgs field, and motion, all particles and energies could be build from these three essential incredients, by giving them different densities of the Higgs field and internal frequencies.
The formula E=(u/k)c explains the relationship of the volume, Higgs field, and motion, within mass.
E=mc^2 is correct and so is E=(U/k^2)c, but if you do some calculations with these two formula arrangements, then you will notice that the two formulas actually will give different answers, depending on the quantities of mass involved.
To illustrate; first use number one in both formulas as the quantity of mass.
Using E=mc^2: M=1, and c^2=9*10^16, then E=9*10^16.
Using E=(U/k^2)c: k^2=1, and U=300,000,000, then E=9*10^16.
In this case the answers are the same for both calculations.
Then use a larger number for the mass quantity.
Using E=mc^2: M=1500, and c^2=9*10^16, then E=1500*9*10^16 = 1.35*10^18.
Using E=(U/k^2)c: k^2=2,250,000, and U=300,000,000/2,250,000, and E=133.3333*300,000,000 = 40,000,000,000.
The larger mass number gives a smaller (potential) energy number.
Then use a smaller number for the mass quantity.
Using E=mc^2: M=0.5, and c^2=9*10^16, then E=4.5*10^15.
Using E=(U/k^2)c: k^2=0.25 and U=300,000,000/0.25, then E=1,200,000,000*300,000,000=3.6*10^16.
The smaller mass number gives a larger (potential) energy number.
In other words, Einsteins formula is correct, it gives an indication of the amount of kinetic energy that is possible for any quantity of mass, but if the mass quantity varies, then the potential energy content also varies and that quantity is not displayed at all with Einstein's formula. The formula E=(U/k^2)c indicates the potential (for) energy in the volume where the mass quantity resides. The less mass there is in a given volume, then there is more potential for the generation of kinetic energy within that volume, and that is one of the things that the formula E=(U/k^2)c indicates.
The above numbers are not precise, they simply indicate the difference in the outcome of using the different formulas.