You received either incorrect or incomplete advice - we don't have the transcript so we can't say for sure. Or maybe this was just confusion and misunderstanding, on the part of either or both parties.

I'm not a lawyer, but I will tell you this - a verbal assurance is not worth the paper it is written on. The written word is better - so I would always first turn to the "primary" source's website, and then make further enquries once I had absorbed this information. I don't believe, however, that the FB agent assured you verbally that buying a, say, V class discount economy fare would allow you to upgrade your flight with miles (You can - though you would need a prior step of paying more to buy up to an upgradable fare class. No problem cannot be solved by throwing money at it). I suspect that what you were told was incomplete, rather than incorrect.

I'm not saying that Flying Blue customer service is a shining example of excellence, and indeed the website too is often incorrect and misleading. However, on the subject of upgrades, the website is pretty coherent and thorough.