Actually the reason why the artstyle changes. Is because this series is basically Ishikawa merging his different manga's (from early 80s to 90s) into one universe.
For example, Tiger of 5000 Light Years is the first entery in the series which was released in 1982 and as you have seen in the last few pages are from that manga.
So you're pretty much seeing his early art style clashing with his later style.