When assigning a new membership to an individual, the join date is capped, I can't set an indvidual's join date as being before 2003.

You can modify the prior years supported in the New Membership form by changing this line in CRM/Member/Form/Membership.php

$this->add('date', 'join_date', ts('Join Date'),
CRM_Core_SelectValues::date('manual', 3, 1), false );

Change the parameter that is currently "3" to "20" — this will give
you current year through 20 years ago on the form.

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