well fixed cups come out same as any other bottom bracket, but i cant imagine any worth his salt frame builder not being able to handle such an issue. i mean after all that is what he does yano. besides whats the worst that could happen youd have to go home and do it yourself which is what your thinking about doing. my experience has always been if i dont have the tools or know how take it to some one who does. goes with anything not just bikes. dont mean to steer you from learning anything but dont want to see you mess up your bike and not be riding. im dealing with a similar problem but its more of frame sag from an under maintained heavily ridden 78 trek but hopefully its saveable i really like the geo on it. but best of luck to you my dude.