Servier Time Zone

Hello,
On 11i version, there is a business need to update the Server Time Zone.

Based on your expertise/experience, would like to know about
- the feasibility of updating Server Time Zone to a different zone (say GMT to GMT-5:00) considering the fact that multiple countries are setup on this instance,
- the scale of impact due to this update,
- ideal period to perform this update,
- any particular features/process to be considered in this process,
- best practices.

Thanks & Regards,
Jawahar.