To replace the original with an extension hose (or the other way around), follow the steps below.
For the record; original and extension hose have the same connections unless you have a hose from before the 2020 model year which used a cubic ring connector around an outlet in the reel core), and thus has a slightly different detachment process. This older type of connection is unlikely to be present on any machines outside of Norway however.
Reel out the entire hose:
In our example below we use an Evolution, the Master has a slightly different u-pin. 
You are now left with an empty drum and easy access to the swivel with a u-pin that locks the hose into the reel core. Use a flat screwdriver or, as in this example, a fork to pull out the u-pin: 
You can now pull out the loose hose:
The next task is to find the extension hose and plug it into the slot you have just opened up. You may need to removed one of the rubber cuffs/collars, depending on the type of hose you have, or to remove the female couplning from the hose connection.