Here are my thoughts.
The only pro is that the users will not require training. That said, training on Unity is very simple and not time consuming.
Cons:
A lot of setup ahead to change the TUI interface to match the NORTEL. Also even if you match the key mapping, the prompts are not going to be the same, so the training pro above is out the window.
Unity is a single digit access for every feature vs Call pilot can be multiple digits (eg, to forward this message press xxxx). So you will actually complicating the use of Unity instead of simpifying.
My suggestion:
Set up Unity in a demo environment and show the people concerned how much easier it is to use than a call pilot, even if your familiarity is with Call Pilot.
My personal experience is that you end up creating more issues that way than solving as the expectations don't meet the results. use the product as it is designed and it will perform much better for your organization.
CISCO supports the key mapping change, so it is not an impossible project, just understand that it will never behave like a call pilot since it was not created to emulate one.