If it's a €300 change fee then it's roughly £100 of fare difference.

While it's true that you will be able to change with a historical fare difference after departure:

1. Ex-VIE was historically very cheap so it's entirely possible that the fare difference to ARN was a lot more;
2. Getting that historical fare after departure will require I-class availability - easy if your trip is a month or longer, very tough if it's a few days.

So unless you're happy to fly HKG-LHR-VIE-ARN on separate tickets, or fly ex-LHR and take the chance on missing the final leg, both of which carry their own risks, the change looks like a very good idea.