When using extended smarty output, the following code illustrates how the
smarty-variables could be used when dropped into the entries.tpl:

{if isset($entry.freetag.extended) && $entry.freetag.extended == 1}
	{if $entry.freetag.tags.tags}
		<div class="serendipity_freeTag">{$entry.freetag.tags.description}
			{foreach from=$entry.freetag.tags.tags item="tag"}
				{$tag}
			{/foreach}
		</div>

		{if $is_single_entry or $is_preview}
			{$entry.freetag.related.description}
			<ul class="serendipity_freeTag_related">
			{foreach from=$entry.freetag.related.entries item="link"}
				<li>{$link}</li>
			{/foreach}
			</ul>
		{/if}
	{/if}
{else}
	{$entry.freetag}
{/if}




Using this subquery you can convert existing categories to tags:

INSERT INTO serendipity_entrytags (entryid, tag)
  SELECT serendipity_entries.id, serendipity_category.category_name
    FROM serendipity_entries, serendipity_category, serendipity_entrycat
   WHERE serendipity_entrycat.entryid = serendipity_entries.id
     AND serendipity_category.categoryid = serendipity_entrycat.categoryid;

[quoted from: http://pixelated-dreams.com/archives/229-Spring-Cleaning.html]

Or using this script you can convert existing categories to tags:

<?php
include 'serendipity_config.inc.php';

$rows = serendipity_db_query("SELECT e.id, e.title, c.category_name
                                FROM {$serendipity['dbPrefix']}entries AS e
                                JOIN {$serendipity['dbPrefix']}entrycat AS ec
                                  ON ec.entryid = e.id
                                JOIN {$serendipity['dbPrefix']}category AS c
                                  ON ec.categoryid = c.categoryid");

foreach($rows AS $row) {
    serendipity_db_query(
        sprintf(
            "REPLACE INTO {$serendipity['dbPrefix']}entrytags (entryid, tag) VALUES (%d, %s)",
            (int)$row['id'],
            serendipity_db_escape_string($row['category_name'])
        )
    );

    printf(
        "Category '%s' added as Tag for Entry #%d, '%s'<br />\n",
        htmlspecialchars($row['category_name']),
        (int)$row['id'],
        htmlspecialchars($row['title'])
    );
}



The following code illustrates on how to change your templates entries.tpl file in case of enabled option 'tags-as-list':

{serendipity_hookPlugin hook="entries_header" addData="$entry_id"}

{if $taglist}
    <article id="taglistentries" class="clearfix serendipity_entry">
        <div class="clearfix content serendipity_entry_body">
            <h2>{$head_subtitle}</h2>
    {foreach from=$entries item="dategroup"}
        {foreach from=$dategroup.entries item="entry"}

            <div class="static-entries-list">
                ({$dategroup.date|date_format:"%d.%m.%Y"}) <a href="{$entry.link}">{$entry.title|default:$entry.id}</a>
            </div>

        {/foreach}
    {/foreach}

        </div>
    </article>

{else}

--- OLD CODE START-and keep --

{foreach from=$entries item="dategroup"}
    {foreach from=$dategroup.entries item="entry"}
    {assign var="entry" value=$entry scope="parent"}
    *** LONG CODE FOR THE entries output article ***
    {/foreach}
{foreachelse}
    {if not $plugin_clean_page}
    <p class="nocontent">{$CONST.NO_ENTRIES_TO_PRINT}</p>
    {/if}
{/foreach}

--- OLD CODE END and keep---

{/if}{* not taglist end *}

--- OLD CODE FOOTER INFO START and keep ---

{if $footer_info or $footer_prev_page or $footer_next_page}
    *** inside CODE ***
{/if}

--- OLD CODE FOOTER INFO END and keep ---

    {serendipity_hookPlugin hook="entries_footer"}