You are correct, matter and energy are interchangeable. That is the whole point of the equation. That is what opened the door to harnessing nuclear fission and (hopefully, in the near future) fusion.
How about the interchangeability of time and money? I'm still trying to perfect that one. Lots of economic entropy involved there ;) Just don't get hung up thinking there is an interchangeability between talent and money. That one will drive you crazy!
For more insight, research the term "binding energy per nucleon". Understanding this will lead to a better understanding of why nuclear fission and fusion are such immense power sources.
In one sense, I am a physicist who took shelter in the steady employment of engineering. I really enjoy it when these types of questions come up. Thanks.