The change stream match expression rewrites for update and replace operations are:

update:  {op: "u", "o._id": {$exists: false}}
replace: {op: "u", "o._id": {$exists: true}}

When a change stream matches both updates and replaces, this will result in a match expression such as

{
  $or: [
    ...
    {
      op: "u", 
      "o._id": {
        $exists: false
      }
    },
    {
      op: "u", 
      "o._id": {
        $exists: true
      }
    },
    ...
  ]
}

This is verbose and inefficient.
Instead, the two conditions can be fused to a single, simpler condition

{
  op: "u"
}

This also allows further optimizations so that the single op type match can be fused with other op type matches into a single IN list match for the op type, e.g.

{
  op: {
    $in: [
       "d",
       "i",
       "u"
    ]
  }
}