Few challenges that you would face and need to add in the planning—
1. Size of current database (this would impact the overall time Upgrade process takes to process and would impact your project timelines…)
2. There might be custom functionalities built in your current Siebel system (due to unavailability in current version). Check if Siebel has included those functionality in the latest version to which you are upgrading. Make an assessment to either continue with the custom functionality or migrating your changes to the new one…
3. Is there a need to migrate from Non-Unicode to Unicode database (it would be a separate effort in the overall Upgrade plan.. Best practice is to first run the Siebel upgrade and then convert the database from non-unicode to unicode)
4. Is there a need to migrate from existing horizontal to latest vertical application

Some considerations if you are upgrading it to 8.0
5. Though Siebel takes care of upgrading the versioned objects (e.g., workflows) in Siebel 8.0 and later, it would be manual effort to re-visit all custom workflows. (One pointer to check in all workflows is Decision steps… Siebel 8.0 made it mandatory to have atleast 1 default connector unlike previous versions)
6. If you have lot of code (scripts), ensure obsolete functions are not used anymore and they are replaced with new / supported functions. Check if you need to migrate your code to use strongly typed variable declaration…