$input = "I am the letter LETTERAAAAAAAAAA like AAAlpha AAAAnd I will always be the first in LETTERAAAAAlphabet.";
$output = preg_replace('/(?:\G|(LETTER))A/', '$1B', $input);
echo $output;

Expected output:

I am the letter LETTERBBBBBBBBBB like AAAlpha AAAAnd I will always be the first in LETTERBBBBAlphabet.

Explanation:

• The \G ensures all consecutive 'A's are matched only if they are part of a previous match.
• The (LETTER) ensures that "LETTER" is not lost in the replacement.
• Each 'A' is individually substituted with 'B' while keeping the exact same length.