Replacing a keyless chuck on a cordless drill

I have a 2nd gen Milwaukee Fuel drill model 2704 and am having trouble replacing the Chuck. I know the correct way to do it, and have removed the left hand retaining screw (torx 40). I used my fuel 1/4 impact and even twisted one bit holder doing it! But now I am using a 1/2 Milwaukee fuel impact to remove the Chuck ( I have a 3/8 Allen socket to a 3/8 to 1/2 socket adapter) the problem is the impact is turning the motor of the drill, so I can not get any real torque on the Chuck. I also heated the Chuck with a high temp heat gun, no dice. I currently have penetrating oil soaking in… see if that helps. If not I might get the grinder out. Oh yeah I got a Rohm rv going in its place, stock Chuck is not impressive. Thoughts?

Impact would seem to be the way to go. I’ve not used your setup - but have used a strap wrench and a mallet break a keyless chuck free on one drill. For another, I used the drill itself in Reverse holding the chuck in a pipe vise.

Never did it with a hammer drill - does it matter how the clutch is set ?

Yeah, I have the drill on drill mode so the clutch will not engage. It seems that most tutorials mention putting the forward/reverse switch in the middle to lock the transmission. However this drill and I believe other modern brush less drills use other methods to hold the transmission while you tighten the Chuck by hand and maybe the impact Is causing the mechanism to disengage? One other thread online was seeming to mention the same problem of the motor spinning with a new brushes dewalt drill.

On my Milwaukees I’ve used a big hex key, tightened into the chuck and hit the hex key with a dead blow hammer. Two hits and it’s loose and you just spin it off.