In a nutshell what needs to be done if your converting from one system to anothr

Hello I'm new here ;D!

I will be part of a Nortel to Avaya VoIP S87xx converstion project, has anyone worked on a similar project?

If so what are some key details or procedures that needs to be done in order to prepare and succeed. 

I understand there are many steps but I'm only looking for overall details, I'm thinking such as:

Extension transfer
DNIS transfer
Trunk transfer
post testing

thank you for your input