Create Transformer Center-tap From 2nd Secondary
You will halve the rated VA of the transformer if you run it at half the rated input voltage.
The rated maximum VA is the product of maximum allowable Volts and maximum allowable Amps. The Amps will be unchanged, that's limited by wire heating. You've halved the Volts, so you've halved the VA, to first order at least. Running the core at half the field reduces the core losses, so the thermal load on the transformer is not quite so high. This means you can increase the thermal loading on the copper a little, at the expense of voltage drop aka voltage regulation. Maybe then the rated VA has only fallen to 55% and not to 50% of what it was.
If you have a centre-tapped 32 V, ie 16-0-16, you can bridge rectify that to about +/- 22 V. If you want +/- 45 V (that is, a total of 90 V from positive to negative, with ground in the middle), then the simplest way to get there is with a centre-tapped 64 V secondary (two 32 V secondaries in series), leaving your transformer configured and supplied as it sounds like it was meant to be, and like my audio amp transformer is.



simulate this circuit &ndash; Schematic created using CircuitLab