Transfomer Connections

It depends on the transformer type, design, winding layout, etc. Unless the transformer is specifically designed to then you should not be swapping your input supply to the transformer's output terminals to convert a step down transformer to a step up transformer (which is what I think you are doing unless your transformer is some sort of autotransformer with primary winding taps).

http://en.wikipedia.org/wiki/Autotransformer

Have you tried the transformer manufacturer's website (or transformer-attached wiring nameplate)?

What's the application and transformer details?

On primary side, if the transformer has sufficient windings to handle the voltage difference.

We would need to know the source voltage and what the actual transformers windings and relative voltages are in order to be able to suggest what possible combinations could be done with it to achieve different output voltages.

For positioning the tab windings, the primary or secondary dosen't matters. But it should be on HT side of the transformer only because of reduced ampere involved.

To do what you suggest would require that you have "tap connections" on both the primary and secondary windings of the transformer that are brought out for this purpose. This is, at least in my experience, an unheard of condition. I have yet to see a secondary on a power transformer with "tap connections".

The purpose of tap connections on a power transformer is to adjust the turns ratio between the primary and secondary to "fine tune" the output voltage of the transformer when presented with a lower or higher primary voltage.

Perhaps, there is something missing in my understanding of your question.