UPDATE (latest): from the looks of it, the SharePoint action "Update item" now only updates the columns/properties which have a new value assigned. Still be aware for the those columns with a default value, as the set value might be reset to the default! Either use an empty value for the "Enter custom value" option or leave this(/these) column(s) out in a dedicated update view which you can select in "Limit Columns by View" advanced option of the Update item action.
As an alternative you could use the 'dynamic' version of Update Item, where you specify a Compose/variable value in the Site URL field (and List Name). As the Update Item action can't determine which content type is to be used, it provides next to the Id field also a Item field. In the latter you can provide key/value (JSON) pairs within curly brackets:
However, it may prove a challenge to update certain data types this way...
UPDATE: for a person/group (single, resp. multi value) it would look like this:
"Requestor": {
"Claims": "@{triggerOutputs()?['body/Author/Claims']}"
},
"TestPersons": [
{
"Claims": "@outputs('Get_item')?['body/Author/Claims']",
"Claims": "@outputs('Get_item')?['body/Editor/Claims']"
}
]
Hopefully this help.
