From what I've read, it was quite common from 3rd to 6th (7th?) edition to use older codexes with the rules set that was supported by Games Workshop at that time. But does it work the other way around as well? Is it possible to use - for example - the rules of 4th edition and the codex of fifth or six edition? And if so, how much tinkering is necessary?
1 Answer
Because the statline format stayed the same, and abilities were written out longhand, the only "tinkering" absolutely necessary would be ensuring Universal Special Rules that appear in the later rulebooks, and are then applied to a unit in the codex for that edition, get backported into the edition you are playing under.
I do want to say, outside of "There hasn't been a codex published for faction this edition", I don't recall it being at all common. The expectation was always that you would use the most recent codex for your faction. Because of the order of publication, and lack of support for some factions, that was frequently from one edition prior, and infrequently more than one edition prior.
