I have a older version European mag release 9 mm P220.
It came with some adjustable target sights (hand rippers) and the fixed welded front sight.
I know I can replace the rear sight since it's dovetailed but question how to change the front without getting into a full refinish.
I'm assuming it has to be milled off.
Suggestions?

You’ve answered your own question. Those sights are integral to the slide and not intended to be removed or adjusted. In addition to milling or some other method such as using a hacksaw, you’d then be faced with the question of how to attach a replacement. I suspect that the original German designers of the guns thought that any required adjustments could be made with the rear sight and therefore the somewhat simpler front sight made perfect design sense.

Well, if you use a competent gunsmith, they will simply mill or file off the front site and then mill in a new dovetail. If done carefully, the only spots where the finish will be damaged is a spot the size of the original front site and the milled out dovetail groove. A simple dab of cold blue put on with steel wool will come close to matching the original finish. And for what its worth, the "bluing" on the earlier Sig's was crap for protection and wear anyway, so you wouldn't be losing much if anything by using a spot of cold bluing especially if this is a shooter which I assume, because if it's a collector, you shouldn't be changing the front sight and instead should put on an original rear to match.

Alternatively, buy a used "folded" P220 .45 slide that has front and rear dovetail cuts, swap in the 9mm breech block and all associated parts in the 9mm breech block (firing pin, extractor, etc) and the original 9mm barrel and it should work fine.

A comment about “gunsmiths.”

Assuming the work you’re discussing is important enough to you to go to the expense and trouble of having it done, keep in mind that it’s not something everyone who advertises gunsmithing work will be capable of doing. Milling a slot for a new front sight would require knowing some very precise dimensions and angles, and having the ability to do that precisely. The experiences I had with several “professionals” years ago was why I learned how to do most gunsmithing chores myself rather than trusting them to someone who just claimed to know how.

Any reputable ’smith who was asked to do that work and knew he wasn’t able to guarantee a satisfactory result would say so, but “reputable” is the key. If you’re going to risk having the work done—and there’s always some risk involved—I recommend you do your research before handing the slide over.