Here is the code I put for assignee changing :
     <?php
                if($ticket->isOpen()) { ?>
                <tr>
                    <th>Assignée à:</th>
                    <td>
    <?php
    if($thisstaff->canAssignTickets()) { ?>
    <form id="assign" action="tickets.php?id=<?php echo $ticket->getId(); ?>#assign" name="assign" method="post" enctype="multipart/form-data">
        <?php csrf_token(); ?>
        <input type="hidden" name="id" value="<?php echo $ticket->getId(); ?>">
        <input type="hidden" name="a" value="assign">
            <?php
            if($errors['assign']) {
                ?>
            <?php
            } ?>
           
             
                    <select id="assignId" name="assignId">
                        <option value="<?php
                                        echo Format::htmlchars($ticket->getStaffId());
                                        ?>" selected="selected">
                        <?php
                        if($ticket->isAssigned())
                            echo Format::htmlchars(implode('/', $ticket->getAssignees()));
                        else
                            echo '<span class="faded">&mdash; Non assignée &mdash;</span>';
                        ?></option>
                        <?php
                        if($ticket->isOpen() && !$ticket->isAssigned())
                            echo sprintf('<option value="%d">S\'auto-assigner</option>', $thisstaff->getId());

                        $sid=$tid=0;

                        if ($dept->assignMembersOnly())
                            $users = $dept->getAvailableMembers();
                        else
                            $users = Staff::getAvailableStaffMembers();

                        if ($users) {
                        //    echo '<OPTGROUP label="Equipiers ('.count($users).')">';
                            $staffId=$ticket->isAssigned()?$ticket->getStaffId():0;
                            foreach($users as $id => $name) {
                                if($staffId && $staffId==$id)
                                    continue;

                                if (!is_object($name))
                                    $name = new PersonsName($name);

                                $k="s$id";
                                echo sprintf('<option value="%s" %s>%s</option>',
                                        $k,(($info['assignId']==$k)?'selected="selected"':''), $name);
                            }
                         //   echo '</OPTGROUP>';
                        }

                        if(($teams=Team::getActiveTeams())) {
                            echo '<OPTGROUP label="Equipes ('.count($teams).')">';
                            $teamId=(!$sid && $ticket->isAssigned())?$ticket->getTeamId():0;
                            foreach($teams as $id => $name) {
                                if($teamId && $teamId==$id)
                                    continue;

                                $k="t$id";
                                echo sprintf('<option value="%s" %s>%s</option>',
                                        $k,(($info['assignId']==$k)?'selected="selected"':''),$name);
                            }
                            echo '</OPTGROUP>';
                        }
                        ?>
                    </select>
            <input class="btn_sm" type="submit" value="<?php echo $ticket->isAssigned()?'GO':'GO'; ?>">
    </form>
    <?php
    } ?>
   
                    </td>
                </tr>
                <?php
                } else { ?>
                <tr>
                    <th>Clotûrée par:</th>
                    <td>
                        <?php
                        if(($staff = $ticket->getStaff()))
                            echo Format::htmlchars($staff->getName());
                        else
                            echo '<span class="faded">&mdash; Non défini &mdash;</span>';
                        ?>
                    </td>
                </tr>
                <?php
                } ?>
To put instead of :

                <?php
                if($ticket->isOpen()) { ?>
                <tr>
                    <th>Assignée à:</th>
                    <td>
                        <?php
                        if($ticket->isAssigned())
                            echo Format::htmlchars(implode('/', $ticket->getAssignees()));
                        else
                            echo '<span class="faded">&mdash; Non assignée &mdash;</span>';
                        ?>
                    </td>
                </tr>
                <?php
                } else { ?>
                <tr>
                    <th>Clotûrée par:</th>
                    <td>
                        <?php
                        if(($staff = $ticket->getStaff()))
                            echo Format::htmlchars($staff->getName());
                        else
                            echo '<span class="faded">&mdash; Non défini &mdash;</span>';
                        ?>
                    </td>
                </tr>
                <?php
                } ?>