As first, we recommend to take a full backup of your project(s) including your existing Eyeshot DLLs.
From version 4 to version 10 many changes were made but with Eyeshot 8 we introduced the Project Converter tool that helps you to migrate from a major release to the next one.
For this reason we suggest to make the migration step-by-step through each version and use the Project Converter where availables.
Here following the suggested steps
(note: after each step, check your project to be sure that it compiles successfully)
v4 --> v5) Check the following article: https://www.devdept.com/Support/Migrating_From_4x
v5 --> v6) Check the following article: https://www.devdept.com/Support/Migrating_From_5x
v6 --> v7) Check the following article: https://www.devdept.com/Support/Migrating_From_6x
v7 --> v8) Install Eyeshot v8 and use the tool as shown in this video.
Here you can read what's changed in Eyeshot 8.
v8 --> v9) Install Eyeshot v9 and use its ProjectConvert tool as decribed here.
Here you can read what's changed in Eyeshot 9 and what are the breaking changes.