hi,
If you can access the secondary winding, usually the outer winding of an 'EI' type transformer, it should be possible to remove some turns from the secondary winding.
At 6A, the wire will be thick, so easy to handle.
Remove 10 turns, measure the voltage, this will give you a rough guide how many turns/volt.
When you know the turns/volt you can calculate how many turns you need to remove for 20Vac.
Its best if you measure the secondary voltage when the transformer is loaded, say about 1Amp.
Always under estimate the turns that need to be removed, you can always easily fine tune the output down by removing an odd turn or two.