Not sure about waiting for so long after the move, might be a good idea so he is more settled and less prone to gastric upset, if that's the kind of horse he is.
As far as changing feed -- do it slowly! That is the rule of thumb. Gradually increase the amount of these new feeds over a couple weeks until you are at the desired amount (for instance with the flax, start at 1/4 of a cup). Personally, during this time I also slowly phase out the previous feed as I am increasing the new stuff, but I've heard of some people just stopping the previous feed all together. IMO I want to make sure all changes are done gradually. Maybe I'm paranoid, but my gelding does have a history of a sensitive stomach so I do everything possible to help him out.
(So, yes, according to my own advice you'd be mixing feed until the previous was completely phased out of his diet and it is just the new stuff).