Insert the MUI2.nsh header file instead of MUI.nsh.
The macros for InstallOptions have been moved to a separate header file unrelated to the Modern UI. If you are still using InstallOptions for custom pages, insert InstallOptions.nsh and use the INSTALLOPTIONS_* macros instead of the MUI_INSTALLOPTIONS_* macros. The macros itself have remained the same.
Rewrite customization code for the Modern UI 1.8 welcome and finish pages in which the InstallOptions INI file is used. nsDialogs commands should be used instead.
Use the standard NSIS method to escape special characters in all texts. For example, $\r$\n creates newline.