Present definition of atomic mass unit

The current atomic mass unit is defined as: $1\,\text{u} = \frac{1}{12}\text{ of the mass of one }{}^{12}\text{C atom}$

Hypothetical new definition

If instead we define the unit as: $1\,\text{u}' = \frac{1}{6}\text{ of the mass of one }{}^{12}\text{C atom}$

then clearly, $\frac{1}{6} = 2\times \frac{1}{12}$ so, $1\,\text{u}' = 2\,\text{u}$

Thus, the new atomic mass unit becomes twice as large as the old one.

Effect on relative molecular mass

Relative molecular mass is a ratio: $M_r = \frac{\text{mass of one molecule}}{\text{atomic mass unit}}$

If the unit becomes twice as large, then the numerical value of relative molecular mass becomes half: $M_r' = \frac{M_r}{2}$

Mass of one mole

We know that numerically, $\text{molar mass} = \text{molecular mass in atomic mass units expressed in grams}$

So originally, if molar mass was $M\,\text{g mol}^{-1}$ then under the new scale, the numerical molecular mass becomes half, hence the mass of one mole also becomes half.

Conclusion

Therefore, the mass of one mole of the substance will decrease to half of its earlier value.

So the correct option is: $\boxed{\text{D: decrease twice}}$