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What I'm getting from what you're asking is that you converted from standard to metric, and now you're wanting it to remain in metric. I would think "save settings" in the file menu would work. Also I don't understand why it would take a long time to convert from standard to metric; if you go to settings/design file/working units there is no converting there is only changing "master units" and "sub units" (a little more user friendly in V8 than SE or J); or are you talking about existing dimension shown on a drawing? I would think that if you go to elements/dimension/units then set to metric and then go to file/ save settings it should remain in metric measuring units.
My Microstation Drawings are set to use millimeters/metric, is there any way I can keep this, but measure areas using metres (as It takes ages to convert each time and there is room for error when converting)