There have been a few documented ways to migrate MT entries to textpattern. One of the issues has been preserving existing links to entries. Here is one way to do it.

[Based on Textpattern 1.0rc1]

1. Find a way to preserve the MT entry id in the custom_10 field in the textpattern table.
   • You can do this manually using an SQL query, or you could hack the migration script to stash the entry value while it's copying entries over.
2. Create a file archives (archives.php if you have content negotiation enabled), just like you would for using clean-urls. This file will map the MT-style archives URL to a new textpattern URL.

Contents of archives:

<?php
    include "./textpattern/config.php"; 
    include $txpcfg['txpath']."/publish.php";
    global $pfr, $url_mode;
    
    if ( preg_match("|archives/(d+).php|", $_SERVER['REQUEST_URI'], $matches) )
    {
        $id = intval($matches[1]);
        $row = safe_row("*", "textpattern", "custom_10 = $id");

        $url = formatPermLink($row['ID'], $row['Section']);
        if ( $url_mode==1 )
            $url .= ( $row['url_title'] ? $row['url_title'] : stripSpace($row['Title']) );

        header('Location: http://'.$_SERVER['HTTP_HOST'].$url, TRUE, 301);
        exit;
    }
    header("HTTP/1.0 404 Not Found");
    exit;
?>