Indeed, the lack of training of agents, and therefore their inconsistency, is frustrating. As you posted elsewhere, the AF culture is not to carefully read all the memos that agents keep receiving.
I would not rely on the opinion of a phone agent (unless you get transferred to the pricing department), either the first or last one that you mention. Pricing done by the system is quite complex and depends on the ticket rules. It could be that one phone agent did read carefully your specific fare rules and has experience. Nor would I rely on your observation that the original fare bucket is not available anymore.. The question is not only whether the same bucket is still available on the return. A higher fare bucket could mean just a slightly higher fare. And the same fare bucket could be used for several return fares. If I is the fare bucket, there can be several I***** return fares. Repricing at current fares depends not only on bucket availability but also whether AF opened new "promotional" fares I*****. or D***** in the higher fare bucket. From HKG, I have seen cases where the fare dropped by more than the change fee. But there is often a rule you cannot benefit from a negative difference (and the refund fee was huge).

As ijgordon pointed out, most fares require repricing the ticket at current fares when changing the first segment. Unless your ticket is somewhat unusual, repricing before departure is not done at historical rate, I strongly suspect that the last phone agent was simply wrong and not aware of the pricing intricacies .