wraptext.php

<?php

// Let's assume the pages in question are named like Blah1, Blah2, etc.
foreach ($pages->find('name^=Blah') as $blahPage) {
    // Let's assume the fields that need to contain HTML are called Html1, etc.
    foreach ($blahPage->fields->find('name^=Html') as $field) {
        // Let's assume these fields have their value in ShouldBeHtml
        $fieldContent = $field->ShouldBeHtml;
        $contentWrapped = '';
        // Parse the field's content as HTML
        $doc = new DOMDocument();
        $doc->loadHTML($fieldContent);
        $doc->normalizeDocument();
        $doc->formatOutput = true;
        // DOMDocument will wrap in html and body tag if it lacks these; I account for this
        foreach ($doc->getElementsByTagName("body") as $body) {
            $unwrapped = [];
            foreach ($body->childNodes as $child) {
                if ($child->nodeName === "#text") {
                    $unwrapped[] = $child;
                }
            }
            foreach ($unwrapped as $child) {
                $body->replaceChild($doc->createElement('p', $child->textContent), $child);
            }
            foreach ($body->childNodes as $child) {
                $contentWrapped .= $doc->saveHTML($child);
            }
        }
        // Save our changes to ProcessWire
        $field->ShouldBeHtml = $contentWrapped;
        $field->save();
    }
}