Pretty basic question. I have looked for [this](https://tex.stackexchange.com/questions/24247/tex-capacity-exceeded-sorry-input-stack-size-5000) on TeX.SE, but it doesn't capture my problem well. Please explain why I am getting this error. Also how to change all the `\&`s to bold ones in the preamble itself?
The problem is that inside your command definition you use the redefined command - this creates an infinite loop.
A simple solution is to store the old definition of the command like this:
ge \& erj