LTG is not Life Time QP membership. Should QF ever decide to remove the free QP membership from SG status, you will be out of the QP (unless - of course - your can gain access through another means).
If QF wanted to do this then anyone already holding LTG would have a fairly good case for compensation.
I would argue this is something like Qantas Club fees (indexed) for whatever period 'life' might be (this might end up having to be determined based on legal argument by the courts, but I would suggest something like 20 years).
While Qantas can alter the terms and conditions and benefits etc with 6 months notice (if they are to our detriment), something which takes a gold flyer 23 years to aquire (14000 / 600 annual renewal) couldn't really be undone with 6 months notice. I would also argue that if Qantas decided to remove LTG all together, there would have to be some cut-off point at which those members already close enough to LTG would have to be permitted to continue to earn SCs to reach that level and its benefits. This would especially be the case for members who could show they were specifically crediting SCs to Qantas to reach LTG.
regards
lme ff