See if you can swap the connector, those pins can be released if you gently lift the locking tabs and pull the wire away from the connector. The metal connectors from your replacement fan might not fit into the original, but it's worth a try.
Thanks for this suggestion! Is there any chance of breaking the connector by doing this? I replied to another commenter, but to emphasize, I am only looking to modify the fan and connector as a last resort because of how rare this equipment is.
Breaking the plastic would be tough so long as you're not hammering it. The connectors crimped to the wires it's another issue and if you pull too hard you could get the wire to come off the connector if it isn't soldered.
Yes you could break the connector if you're not careful. However at the end of the day, it's not like you can't replace both sides of the connector with standard hardware, and at the very worst you could solder the fan to the pins and cover each wire with heat shrink.
Why not just cut the wires and solder the connector to the new fan.
I am trying to be non-destructive since this is a rare piece of equipment. So I would prefer to put in something entirely new in and preserve the original hardware in case something goes wrong.
Any chance to replace the whole PSU and then going full tilt modifying the replacement? I only see one offer for 400-5494-91 (400€!!) so maybe any other in that form factor? Seems to be mostly standard PC connectors on these, but not sure from the pics I can find.
One possiblity would be to cut the original cable and add a new connector to both sides. A female one to the one that goes to the board, and a male one to the one on the fan.
That way you have all the original parts still there, but you can use the one part of the cable as an adapter.
Buy the JST plug with the right pin pitch (in your case 2mm), and grind it so it fits. You can find male to female cables, so you also don't have to modify the original connector on your noctua fan.
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