I bet you that if you hit Edit on the Relationship [someone previously not inheriting the Membership] - and then Save (without changing anything) - they now do inherit the Membership as you intended right?
So the Q is not just how do you change the maximum number of membership - but it's really about how do propagate that across.
You need to automate the trigger - I'm thinking an extension that essentially does an update Membership without updating the Membership.
Ok we can test that in the API explorer!
I just tried this on one of my sites:
$result = civicrm_api3('Membership', 'create', array(
'sequential' => 1,
'membership_type_id' => "Fort Smith Kayak Club - Family",
'contact_id' => 2,
Trying to update my own Individual Membership to a Family Membership - and that worked in that I inherited all my kids on this Family one.
Point is - updating the Membership via the API triggers a 'check to see how many should inherit'
So I recommend a small custom extension that loads Memberships and 'updates' them. That should work!