One way to normalize one particular type of multi-dimensional array.

multidimensional_normalizer.php

<?php
		return array_merge(
			... array_map(
				function( $key, $values ) {
					if ( ! is_array( $values ) ) {
						return [];
					}

					return array_map(
						function ( $value ) use ( $key ) {
							return [
								'key' => $key,
								'old_value' => $value
							];
						},
						$values
					);
				},
				array_keys( $post_meta ),
				$post_meta,
			)
		);