If you have an adaptaplug, you can turn it around. Check polarity with a voltmeter.
If you've got a fixed adapter, yes, just hack the cable.
One trick I do is put a bit of red shrinkwrap on the plug if it's polarity if reversed. I have a few wallwarts that were either reversed already, or hacked by me. So I "flag" them with a red band around the plug to indicate they are for UA5, R4, etc.
Richard