Yes, I've since read other comments and have a better understanding of the reasoning behind .attr and .prop, so I'm a little less bah-humbug about having to refactor before upgrading. It appears that search/replace .attr('value') => .val(), .attr('checked') => .prop('checked') are the big offenders, but probably need to look closer at .attr('enabled') and .attr('disabled') as well.

I have this code:

  $.fn.setAttr = function(attr, bool) {
    if(bool)
      this.attr(attr, attr);
    else
      this.removeAttr(attr);

    return this;
  }

  $.fn.disabled = function(bool) {
    return this.setAttr('disabled', bool);
  }

  $.fn.checked = function(bool) {
    return this.setAttr('checked', bool);
  }

I assume I should change all attr to prop?

I think you should add two more helpers (like .val):

.checked() and .disabled() - with no parameters they return a boolean, and with a parameter you can pass in true/false to set the property.

Those two are probably the most commonly used properties and if you had them most people could probably ignore attr vs prop.