How many meaningful words can be made by changing only the consonants in the word MEAN so that each of the consonants is replaced by the previous letter in the English alphabet, by using each letter only once in each word?

Let's break down the given word: MEAN
* Consonants are: M and N
* Vowels are: E and A

According to the rule, we change each consonant to its previous letter in the alphabet:

* M $\to$ L
* N $\to$ M

The vowels remain unchanged:

* E $\to$ E
* A $\to$ A

Now, the available set of letters is: L, E, A, M

Using these four letters exactly once per word, let's look for meaningful English words:

1. MEAL (A valid English word meaning food eaten at one time).
2. MALE (A valid English word relating to the masculine gender).
3. LAME (A valid English word meaning unable to walk properly).

Thus, exactly three meaningful English words can be formed.