I guess the question that arises is whether the additional 6 months is an extension to re-qualify or is a renewal. If the former, if a person had accumulated 8k miles and 3 A3 segments, their profile would show that they need 4K and 1 A3 segment to re-qualify. If the latter, the profile would show just 12k and 4 A3 segments. Unfortunately, because my friend had accumulated no miles and no segments, we get no help.
If the 6 months is a renewal, I see your point that paying the 39 Euros does appear to get you an extra 6 months.
However, if the 6 months is an extension to re-qualify, you are actually ahead by a further 6 months, presuming you can accumulate the additional 3k during the period (renewal at 12k, not 9k).
Note: My friend’s profile shows she has 6 months to accumulate 12k and 4 A3 segments. If this was a renewal, I would think they would have given her the normal full 12 months to re-qualify for the following year. Hence, this might just be an extension to re-qualify.