SRENNAPS, when I joined up in 1968 as a rigger craft apprentice I was told that my career would, if I did my bit and wanted to stay, enable me to stay to age 55, due to the reserved rights you correctly pointed out.
The career path was J/T to Cpl in 3 years, Cpl to Sgt 4 years later and Sgt to Chief after another 5 years, which should have been chief by the 12 year point.
Due to changes in the rules in 1980 I lost my reserve rights (this was my 12 year point and this was when I became a SNCO), this with the later (approx 1990) requirement to be a Flight Sgt by age 47 was what meant I could not continue to age 55.
I pointed out that these changed my original 'contract' and I was baffled by admin bulls**t with regards to the 'new rules', at one point I was informed that it was my fault I had lost my reserved rights, did I persistantly question and fight the system on these and the technical issues as well up through the chain of command - yes, with the result that I remained a Sgt for 16 1/2 years which obviously stopped me achieving Flt Sgt, I did get my Chiefs though so I got what I had worked for in the end.