Don't see why not - as I read it, basically whatever you have accrued and the rights that go with it remain the same until the switch to the new scheme. This means that you can draw your IP from 16/38 as normal, and thus by inference should be able to commutate your gratuity as normal.
Whether or not HMT puts a spanner in the works to stop it or make it an unattractive proposition through changes to the tax regime is another matter entirely.