Event sourcing modeling problem

Member.php

<?php

/*
* A bit of background:
*
* Because this is a project where one of the main goals for me is to learn new techniques, 
* I want to apply event sourcing to this project.
*
* We're modelling  a student association where we want to manage memberships of the association.
* Most members will finish their studies in about 4 - 6 years and if they do not stay active 
* within the association, then they will be noted as former members.
*
* A key difference between a member and a formerm member (could) be that a former member
* can't join committees and it will leave all committees once it becomes a former member.
*
* I'm  curious about different ways to model this. I quite like the idea of using
* different classes for the different states of a member (inspired from: http://www.slideshare.net/jeroenvdgulik/2015-0415-object-invariants-immutability-you-njjmegenphp),
* though I'm not too sure if this is a good practice when using event sourcing.
* Morevoer I would have to change some of the methods of Broadway's EventSourcedAggregateRoot 
* so that they will always return the current (or new eg FormerMember) model
*/

class Member extends EventSourcedAggregateRoot
{

   public function reconstitute(EventStream $stream)
   {
       $member = new static;

       return array_reduce($stream, function($member, $event) {
           return $member->handle($event);
       }, $member);
   }

   public function becomeFormerMember()
   {
       // apply will call applyMemberBecameFormerMember 
       return $this->apply(new MemberBecameFormerMember($this->id));
   }

   private function handleMemberBecameFormerMember(MemberBecameFormerMember $event)
   {
       $this->leaveCommittees();
       return new FormerMember($this);
   }
   
   public function joinCommitte(CommitteeId $committee)
   {
       // do stuff
   }
}

class FormermMember extends EventSourcedAggregateRoot
{
    // does not contain joinCommittee
}