Transformer turns and effect on frquency
Put simply lets consider an inductor.  Transformers are just coupled inductors.

Some of the key relationships are:

$$V = L \cdot \dfrac{d}{dt} I = N \cdot \dfrac{d}{dt} \Phi = N \cdot A \cdot \dfrac{d}{dt} B$$

Where: 

\$ A \$ is Area (square meter)

\$ B \$ is Flux density (Tesla)

\$ I \$ is Current (amp)

\$ L \$ is Inductance (henry)

\$ N \$ is Turns

\$ V \$ is Voltage (volt)

\$ \Phi \$ Flux (weber)

\$ \dfrac{d}{dt} \$ is rate of change with respect to time, differential.

If we take transformer then and reduce the frequency we are in effect increasing the \$ dt \$ so all other things being equal you would expect to see an increase in both the flux density and the current.  The best you can hope to see from this would be for the transformer to run hotter because it will have higher core and copper losses.

All other things are not equal however and high frequency magnetics are designed to run at certain flux densities: Too high and they risk saturation, too low and the part is over-designed and costs more than it needs to.

As you reduce the frequency to maintain the optimum flux density swing you will either need to increase the the effective area of the core or add more turns. In practice this usually means an increase in both and requires a larger, more expensive core.