I don't think any of them were really proper replacements for any of the others. Animal Magic finished in 1983, while Wildtrack ran from 1978-85, and The Really Wild Show started in 1986. Animal Magic and Wildtrack were very different to each other - the former was based around the kind of animals you'd find in a zoo, whereas Wildtrack focused on things like toads, badgers and otters. I suppose RWS kind of combined the two.
Yeah, The Really Wild Show was probably more a replacement for Wildtrack than Animal Magic. What is the case, though, is that you used to get two series of Animal Magic a year - a "proper" series in January and then another series in the summer, often the only new kids show in the entire week, which would be made up of compilations, re-edits and specials, like that episode when Johnny and Nutkins did that CSO 'safari'. When Animal Magic ended, the last official series was in January 1983, then there was a summer series which mostly starred Nutkins and then in January 1984, when you'd usually get Animal Magic, Wildtrack had been moved to January from its usual Spring slot to fill the hole (although there were a few more ooutings for Animal Magic, a compilation on Good Friday 1984 and then repeats in the Easter and Summer holidays).
Wildtrack had a slightly more journalistic approach, in Anna Home's book she talks about them running a feature early on involving a postcard which was supposed to illustrate the beauty of Scottish wildlife but some of the animals in it were blatantly stuffed. It was created and produced by Mike Beynon who also went on to produce the last few series of Animal Magic and I assume there was some sort of clash between his more journalistic and academic approach and Johnny's views. One reason why he didn't do the new series he was offered when Animal Magic ended, according to The Times, was because they wouldn't let him choose the producer. And I've got the 21st anniversary Animal Magic from 1983 where Nutkins does a joke about the producer ("this is such a special day even the producer can have a bit of this cake!") which Johnny finds terribly amusing.