Escape Parameters with [ in name.

{form type=hidden name=default_address[address_class] value=default}

New:

{form type=hidden name='default_address[address_class]' value=default}

Item/data not same name

Old:

{foreach name=aussen key=key item=data from=$data}

New:

{foreach key=key item=product from=$data}

Update Include Paths

{include file="/xtCore/pages/checkout/subpage_shipping.html" type="tpl_include"}

New:

{include file="$selected_template/xtCore/pages/checkout/subpage_shipping.html" type="tpl_include"}

Updates required in Source Code:

Not Supported anymore:

DB Template Resources like:

$this->Template->registerResource("db", array (
$this,
"resource_db_source",
"resource_db_timestamp",
"resource_db_secure",
"resource_db_trusted"
));

Solution:

move to new smarty3 Custom Resources http://www.smarty.net/docs/en/resources.custom.tpl
if you just want to fetch templates from a string, you can use string resources instead ob current db: resource http://www.smarty.net/docs/en/resources.string.tpl

Updating Template for Smarty 3