We know that the current that the electron will be emitting when it is in it's original state is e/T, or which is ev/(2*pi*r).

So, the field at the centre will be (mu*e*v)/(4*pi*r^2) = B.

Now, from this relation we can easily observe that B is inversely proportional to n^3. So, if the electron jumps from n=1 to n=2, then the magnetic field will reduce by a factor or 2^3 or 8. So, the new field is B/8.