Questions about Replace with regular expression and excepting specified filed

At first, sorry for my poor English.

I have tried to understand but I can't still now.

I have 2 questions.

Q1. I want to change

AA@55555@BB@666666

to

AA@@@55555_BB@@666666

with Replace with regular expression.

A, B is random character A~Z.

5, 6 is random number 0~9.

@ means space.

How can I change this with Replace with regular expression?

Q2. I want to use Actions(e.g: Case conversion, Replace, etc..) in all field except some specified fields I want.

How can I except these in any actions?

Help me please.