The 6v and 12v V3s are identical physically, they're simply calibrated differently. What battery did you use for your UA5?
A 6v V3 will run on a >= 6v battery. Only catch: the low battery indicator and auto-shutoff features may not work properly because they're calibrated for a 6v SLA battery - specifically, low battery indicator set to trigger at 5.75v and auto-shutoff at 5v. If you're using a battery with a low battery indicator, like the popular li-ion batteries, it's no big deal and not really necessary to recalibrate.
A 12v V3 battery will run on a >= 12v battery. It will not run on a battery < 12v (e.g. 9v li-ion) because the low battery indicator is set to trigger at 11.5v and auto-shutoff at 10v. In this case, you'll need to recalibrate.
To recalibrate, see the
V3 voltage calibration PDF from Grace Design.
Bottom line: 6v should work straight away; 12v should also work, but may require calibration.