As someone who has actually has a bit of knowledge around this, keep in mind that there is a lead time for stuff being written, and a codex will typically be finished some months before it is released. Anything he has had a direct hand in, that influence should be gone within, say, a year after he has left. After that, anything should be brand new.
Until recently the turnaround on a codexes has been about once per edition (less for some, true), which has meant a codex lasting about 5 years. The transition from 6th to 7th, and the rise of the digital supplements is changing that, so we could in theory see a brand new codex quicker than that now.
What way a given codex is going to go depends on the design philosophy of the studio, and probably what the suits think will sell, at the time. Sometimes this works, sometimes it goes horribly, horribly wrong, and you're stuck with a pathetic excuse for a codex for years.