At the time I purchased by Triban bicycle i was not sure between the 520 and the 540. The salesperson told me that I could buy the 520 and if I wanted I could exchange it for the 540 as long as it was within 1 yr, the bike was clean and everything was functioning. This comment surprised me as I could not imagine what they would do with a used bike. On a separate visit, I query this policy and they told me the very same thing. Having been using the 520 for 4 months, I'm overall very happy with the bicycle, but the gear set is the lowest end of shimano and it is not as good as the 540. Today (4 months after I purchased it), I tried to exchange it and surprise surprise, they did not honor what they had told me before. They said the bicycle was used and it was not in perfect state (of course it is not I have been using it - normal wear and tear - everything functions perfectly). I understand they won't exchange used bicycles as they cannot re-sale them, but they should be honest about it when they sold it and again on the later visit. Note Decathlon will not take your used bicycle back even if they tell you they will.
It doesn't say that anywhere it just says that as long as you are not happy with it then you are free to do so.
From what I can see you've not really mentioned any reason you are not happy with it other than the Spec on the 540 is better. Also Sora is not the lowest spec in terms of the Shimano hierachy, it is infact Claris.
Personally I would exchange it, as would most stores. We do however have some who are very fussy in the term of exchanging used products.