Hard to read. Too long boolean expression.

.

yes, Strings are always immutable (unchangable)

But now you do...

Massive W to all Java devs on StackOverflow

And is correct that way because inputs "... are always lowercase strings, and that each has at least two letters.". If that was not the case and arbitrary inputs would be allowed we'd also need checks for null and lengths in order to avoid NullPointerException and IndexOutOfBoundsException or alike.

it work only if cases are equal

I would rather have more spaces to increase the readability of the code, but it's okay

Actually IntStream.of() calls Arrays.stream()
From IntStream.java :

<...>
/**
 * Returns a sequential ordered stream whose elements are the specified values.
 *
 * @param values the elements of the new stream
 * @return the new stream
 */
public static IntStream of(int... values) {
    return Arrays.stream(values);
}
<...>

I googled, and may be understood the patterns problem you mentioned. But this version is same as:
replace(char oldChar, char newChar) right?