Hi. I think I would suggest using a Relational Grid control for both the original selection and the refinement. When the user first selects the values, the grid would be configured to show all available values (essentially, use a listing report that reports all valid values, Show only selected items=no, Display 'Show only selected items' checkbox=(your choice). When the user edits the same multi-relational field again on the 2nd transition, the grid could be configured to still show all available values (essentially, use a listing report that reports all valid values, but Show only selected items=yes, Display 'Show only selected items' checkbox=never.

If you needed something more advanced, you could potentially create a runtime report instead of using a report definition in Composer to support the grid. You could even make that report an advanced SQL report if you needed to do even more advanced filtering (such as relating values between two multi-relational fields.).

Is this just a visual change?  So that users see just the selected values but with a "checked" checkbox next to each??  Is it OK if the check box is grayed out?? Otherwise the user can check/uncheck it.  That would be even more confusing. 

I'm not using checkboxes, however I do hide values from item type via javascript

var myField = jQuerySBM(GetFieldByName("Item Type"));
myField.find("option:contains('Retire')").remove();
myField.find("option:contains('Impact Subtask')").remove();

I can't understand if you want to change the State form or the Transition form.  I've got some JS that does both.  This will add a Checkbox to the multi-relational field on a State form so that it looks more like a transition form.  The checkboxes are disabled because we don't want the user clicking on them thinking that might do something.  Put this in a JS widget on the form.  You'll need to supply the field name of the multi-rel field in the call to "Add_Chkboxes_to_MultiSel_on_State_Form("<field name here>");".  In my demo screenshots the multi-relational field is called "Installation Environments"

In Composer, the Multi-rel field settings are:

Options / Style: Checkboxes

Options / Search and query: Appears in report field lists; Show full list

Forms are "Standard layout" and NOT "Legacy mode".

function Add_Chkboxes_to_MultiSel_on_State_Form(strFld) {
let msel_fid = _LookupField(strFld)?.fid ;
if (msel_fid) {
// find the SPAN elements contained in the specified field's WRAPPER.
let spans_elts = document.querySelectorAll("#" + msel_fid + "\\.wrapper span") ;

if (spans_elts.length > 0) {
spans_elts.forEach((elt,idx) => {
// console.log("elt=",elt,"; idx=",idx);
// let new_elt = document.createElement();
// elt.before( new_elt ) ;
// insert a Checked checkbox before the element's text.
elt.insertAdjacentHTML("beforebegin",'<input type="checkbox" id="' + msel_fid + '.chkbox' + '" checked="checked" disabled="disabled">');
elt.insertAdjacentHTML("beforebegin",'&nbsp;');
});
}

} else {
alert("Can't find field '" + strFld + "'");
}
}

Add_Chkboxes_to_MultiSel_on_State_Form("Installation Environments");

Here's another function for a Transition form that will hide the elements that are already selected.  I don't know why you'd do this because it prevents users from "deselecting" an environment -- they can only add environments.  Again, you'll need to specify the name of the multi-rel field.

function Hide_Selected_MultiSel_items_on_Xstn_Form(strFld) {
let msel_fid = _LookupField(strFld)?.fid ;
if (msel_fid) {
// find the "checked" INPUT elements with the specified field ID contained in the specified fields WRAPPER.
let spans_elts = document.querySelectorAll("#" + msel_fid + "\\.wrapper input#SELECT_" + msel_fid + "[checked]" ) ;

// Delete, hide or ignore :
// Hide the "checked" INPUT element.
// Hide the LABEL element that follows it.
// Delete the text that is part of the LABEL element.
// Ignore (skip the SCRIPT element).
// Delete the "  " text.
// Hide the ANCHOR element that follows that.
// Delete the IMG tag within the ANCHOR.
// Delete the BR tag that concludes the line comprising the Multi-sel Aux table record.

if (spans_elts.length > 0) {
spans_elts.forEach((elt,idx) => {

/*
console.log("INPUT elt=",elt);
console.log("LABEL next=",elt.nextSibling);
console.log("#TEXT !!next=",elt.nextSibling.nextSibling);
console.log("SCRIPT next=",elt.nextSibling.nextSibling.nextSibling);
console.log("TEXT !!next=",elt.nextSibling.nextSibling.nextSibling.nextSibling);
console.log("ANCHOR next=",elt.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling);
console.log("IMG !!next=",elt.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling);
console.log("BR next=",elt.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling);
*/

// Hide the hidable elements.
// Get references to the elements that can't be hidden and will be deleted.
// Don't delete elements that HTML will send back to server as part of the list of items selected.
elt.setAttribute("hidden","hidden");
elt.nextSibling.setAttribute("hidden","hidden");
elt_textnode = elt.nextSibling.nextSibling;
// elt.nextSibling.nextSibling.nextSibling.setAttribute("hidden","hidden");
elt_txt = elt.nextSibling.nextSibling.nextSibling.nextSibling;
elt.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling.setAttribute("hidden","hidden");
elt_img = elt.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling;
elt_br = elt.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling.nextSibling;

// Now delete the elements that we can't hide. I'm doing this starting with last element within the HTML to be removed.
elt_br.remove();
elt_img.remove();
elt_txt.remove();
elt_textnode.remove();

});
}

} else {
alert("Can't find field '" + strFld + "'");
}

}
Hide_Selected_MultiSel_items_on_Xstn_Form("Installation Environments");

Here's screen shots of State and Transition forms "as-is" and after running the appropriate JS.

I pasted the code with the "Format / Inline / Code".  That seems to have removed all the leading whitespace.

The JS is probably more-or-less hard-coded to the HTML used to display Multi-Relational fields in SBM 11.8.  I don't know how the HTML changed from version to version.