If your UPO is gaining a few seconds a day, I would strongly suggest that you leave it as it is. If your watch with genuine eta movement is running that well, it is within COSC specs and I would be very suspect of any watchmaker willing to take your money to even look at it.
The chances of movement that is already that accurate, any more accurate are fairly slim.
NOW, if you are 15+ seconds fast, then the simple turning of the adjustment screw should get you down to where you should be. Again, an honest watchmaker would most hopefully tell you to leave it alone!
If you do find someone who wishes to make the adjustment, they will most likely charge an hours rate to even open the watch. Typically this would be in the area of $35-$50.