I think contributory pensions might have been discussed a while back as part of broader MOD finance issues, along with abolition of increments. Not heard anything since and if I were running things I would be very nervous about any such move given the already tight - and in some cases parlous - manning situation. Plus pay is already abated to take pensions into account. Things aren’t going that well that people will be willing to lose even more money on top of already reduced pensions and years of pay restraint.
The easy way round this is just to put everyone on the same scheme. I suspect the numbers of personnel remaining on grandfather rights are relatively small and at the tail end of careers making the impact if they walked a little more baresble. It would be significantly easier than coming up with yet another new scheme - the third in 15 years.