That sounds like a good change to me. IIRC, neither Nutrena nor Purina publish their NSC levels (sugar + starch) for their feeds, but I believe Safe Choice is lower NSC, and therefore a healthier choice overall. I like Triple Crown better, since they do publish their NSC levels and also have fixed formulas (which are also published online), but TC can be hard to find in some areas.
As far as selenium is concerned, that would be a good question for your vet. S/he should be familiar with the local selenium levels. You can help the vet out by doing a little pre-work by calculating how much selenium you'll feeding from the grain and supplements. Extra credit if your hay has been analyzed
The key thing is to make changes slowly. Start mixing in the Safe Choice with the textured feed, slowly increasing the amount of Safe Choice and decreasing the textured feed, and switch over completely over the course of a week or two. If you're keeping the amount of Amplify the same, no need to stop it while you're changing the grain. If you're increasing or decreasing, do it slowly as well. Same with the Magic Cell.
I'd go ahead and do the Power Pac- I'm guessing your vet suspects a heavy worm load could be causing the anemia, so you'll be fighting an uphill battle with the Magic Cell until the worms are cleared out.