Programming :: Substituting Variable Upon Cat Execution?

I have a file named "test.txt". It contains the following:

Hello $USER1
Your boss is $ADMIN1

Now, I want to do something like this:

export USER1="World"
export ADMIN1="Me"
cat test.txt > test2.txt

And achieve something like this when I view the contents of test2.txt:

Hello World
Your boss is Me

The problem is, it's not that easy to achieve. I don't want to do the "echo Hello $USER1 >> test2.txt" because I'm planning to implement this on a MB-sized text file.