I've been trying to figure it out but am stumped so far, hoping someone else has run across how to change this behavior.
I think the following code from the profile_fields.tpl file defines the checkbox and have been looking for the fn_show_tag function definition thinking it may hold the answer, but haven't tracked it down yet.
{if $shipping_flag} <span style="padding: 15px 0px 6px 0px;"> <input type="hidden" name="shipping_eq_billing" value="N" /> <input class="valign" id="seqb" type="checkbox" name="shipping_eq_billing" value="Y" onclick="{if $profile_fields_equals == 'Y'}fn_show_tag('sa',this.checked);{/if} fn_copy_fields(this.checked);" {if $shipping_eq_billing == 'Y' || $mode == 'add'}checked="checked"{/if} /> <label class="valign" for="seqb">{$lang.shipping_same_as_billing}</label> </span> {/if}
Any help would be greatly appreciated.