Is there a simple circuit that can change the AC output voltage from a transformer or AC-AC power supply?
The component you're looking for is..  another transformer. 

Are you looking for "9v a.c" RMS? Peak-to-peak? What does the input to the device you're trying to power look like (electrically)? 

It's not trivial to adjust the voltage of an AC signal after transformation;  but it's generally pretty easy to simply get a new transformer. 

You can try disassembling the power supplies and checking if they have an additional tap that happens to provide the voltage you're looking for (which is rare in low-cost parts;  but you can often salvage multi-tap transformers out of other equipment!),  or simply replace the transformer inside with one that has the correct turns ratio. 

Alternatively;  depending on how long you need such a solution;  you could also simply power either existing power supply with an Autotransformer; (A device that simply allows you to "tap" the output anywhere along the coil,  giving you a variable turn ratio transformer) to step down the voltage going into the power supply before it steps the voltage down again.  (This is NOT efficient;  but for testing and temporary use;  would also allow you to convert the incoming power to the output you're looking for.)



If you're feeling very adventurous (I'd HIGHLY recommend against this unless you really know what you're doing!)  you can remove some turns from the secondary side of the transformer to get closer to the voltage you expect (though you WILL de-rate the total power capacity quickly!)   Depending on the rest of the power supply;  any AC supply should run at a lower voltage without a problem.