What is the term for replacing a letter with a number in a word, such as UNREAL being written as UNR34L? Quite often seen on licence plates, it is almost satirical misspelling, but the definition of that does not seem to include substitution of letters for numbers.
I think the correct term is "leet" or "leetspeak" used as a noun. I've never seen it used as a verb but you probably could.
"leet" is a contraction of "elite" and is often written "l33t" and it hearkens back to when people would express their own technical superiority in online chat rooms by using leetspeak and thereby making it harder for n00bs (new users) to understand what's going on.
Leet consists of much more than the substitution of numbers for letter, though that is a big part of it, it is a fairly complex code with its own vocabulary, thereby being quite effective at excluding the uninitiated.