Before being compiled into the 45 volumes we know today, each chapter of Doraemon was published separately in various magazines for children of each age, from kindergarten to Grade 6. Different magazine means different art styles and stories appropriate for each age. Chapters where Doraemon has his usual appearance are normally published in magazines for 3rd graders and older - this chapter was published in the magazine for 2nd graders. So it's not that the author changed his art style half-way through making a volume, it's because each volume is compiled using many chapters from different magazines, and not even in a chronological order!