I think that if you take care to open the casing carefully (watch out for the spring system) it should be possible. I've had 2 of these for more than 10 years, and as winter approaches I take them in and periodically, once they're completely unrolled, I spray the inner shaft and the winding system with VD40.

But has anyone ever opened it?

Hello
No idea, it's never happened

In my opinion, the cost of the replacement tube with the integrated drum will be more expensive as a spare part than the complete system on Galaxus. You can also contact Gardena service for official information.

My guess is, that it is not possible to replace it. At least there was no instruction in the manual.