Difference between 8th and 9th Editions for A First Course in Differential Equations by Dennis Zill?

In most instances, the only change you will find is that errors in earlier editions get corrected in the later addition. Occasionally, in math books, the study questions may be changed. I do not know specifically about Zill. In general, for textbooks like this, I bought older additions (cheap) and would check in the library or a friends copy of the new edition if I had any question. The occasional situation will arise where a new chapter is added, and I would arrange to photocopy that material if necessary.