Formio.js get value of select component before and after change

I use formio javascript library to create my forms and I simply need for a dropdown to get its current value and its previous value on change event.
The challenge here is that the dropdown can be in a datagrid and more dropdowns are added when adding row. Like with this simple form configuration:

{
“display”: “form”,
“components”: [
{
“label”: “Data Grid”,
“reorder”: false,
“addAnotherPosition”: “bottom”,
“layoutFixed”: false,
“enableRowGroups”: false,
“initEmpty”: false,
“tableView”: false,
“defaultValue”: [
{}
],
“key”: “dataGrid”,
“type”: “datagrid”,
“input”: true,
“components”: [
{
“label”: “Test drop down”,
“widget”: “choicesjs”,
“tableView”: true,
“data”: {
“values”: [
{
“label”: “Test 1”,
“value”: “test1”
},
{
“label”: “Test 2”,
“value”: “test2”
},
{
“label”: “Test 3”,
“value”: “test3”
}
]
},
“key”: “testDropDown”,
“type”: “select”,
“input”: true
}
]
}
]
}

Can you help me?
Thanks