$match stages appended to a change stream pipeline can use a custom collation for comparisons, but the oplog scan always uses the simple collation (see SERVER-31443). When performing rewrites, we must check whether the pipeline collation is non-simple; if so, we must abandon the attempt to rewrite the filters, since these may be more restrictive than the originals. For instance, {$match: {"documentKey.field": "blah"}} with a case-insensitive collation should match both "blah" and "BLAH", but rewriting this into the oplog will only return exact matches on "blah".