My guess is that in the US the IRS would want the sale of metal to be taxed if it resulted in a gain from it's basis. The fact that one immediately put the money to use buying another metal would be a new transaction... starting the basis for that metals sale in the future.
As vertical1 pointed out though... whether the IRS knows about it or not would be a consideration.
Trading one ETF for another ETF would certainly leave a trail and the IRS would presumably view it as stated above. Trading with your neighbor, or if you find a venue that does not report, would be a different thing.
rutherium, what do the charts say about palladium currently?