Tell the PMPro Proration Add On that every level change is a "downgrade" or sidegrade.

pmpro_is_downgrade_true.php

/*
  Tell the PMPro Proration Add On that every level change is a "downgrade" or sidegrade.
  
  This will cause the PMPro checkout to always charge $0 for level changes, while setting up
  a subscription to charge the new plan price on the next payment date.
  
  This is useful if you have all recurring membership levels that are basically payment plans vs
  differently priced levels.
  
  Add this code to a custom plugin.
*/
add_filter('pmpro_is_downgrade', '__return_true');