Transformer Adaptation

Unless you are in the transformer design and rewind business I would suggest that you get the proper transformer. There's more to this than simply adding or removing a few coils of copper.

You can replace with the spare one. Use series reactors to manage the voltage differential to an acceptable value. The series reactors will additionally limit the short circuit current and are extensively used.

Although I agree that series reactors can be used to limit short limit currents, please explain how a passive series reactor will "...manage the voltage differential..." since the desired voltage drop will vary as the load through it and the stiffness of the system to which it is attached. A better(?) solution would be to lower the output voltage of the generator to 10.33kV so that the high-side voltage of the replacement transformer remains at 132kV. Of course this ignores the problems of fitting a 420MVA transformer into the space previously occupied by the 115MVA one, the possible impedance differences between the transformers and its effect on system voltage regulation, as well as the potential derating of the generator due to its inability to reach full load current at the lower maximum voltage along with the required retuning of the Automatic Voltage Regulator system. There's more to this question than meets the eye.

I said "series reactors are extensively used" and if so load variations occor in each and every case, still the systems are manageable.

As for the OP, adjustment of the speed droop of governer will keep the machine steady, while for you the relation of induced EMF per coil is

(weber flux pole * P/60) X (120 * f / P)

and cannot be altered as desired.

Please provide an actual example of your proposed series reactor solution. Using random cut and paste factoids only serves to obfuscate not enlighten, the resultant lack of knowledge transfer is not a win-win.

Dear Ram you are at liberty to start a new thread, as for this space it is the domain of OP and all responses should be relavant to the OP's concerns.