No.  There are no major technology changes needed to move ArcGIS Desktop or ArcGIS Engine applications to ArcGIS 9.3.

ArcGIS Desktop and Engine applications should be recompiled against ArcGIS 9.3 to ensure compatibility, but no code changes are required.

Existing Map documents and data can be directly used in ArcGIS 9.3 without conversion, and you may incrementally migrate clients from 9.2 by installing Service Pack 5 (or later) and the ArcGIS 9.3 geodatabase direct connect for 9.2 clients patch downloadable from ESRI’s support site:

http://support.esri.com/index.cfm?fa=downloads.patchesServicePacks.viewPatch&PID=66&MetaID=1416