Basically, I wondered what the best method was to replace one item in an immutable list. I first used the included Replace<t>(T oldItem, T newItem) "native" method (got the oldItem through Find() or FirstOrDefault)), then also tried the overload with the IComparer and then tried your extension method which has to call .toImmutableList() at the end as Select() only returns an IEnumerable.

Created a basic ImmutableList with 10,000 entries (each entry being public record TestRecord(string Id, string Content)) and ran it through a Benchmark when trying to replace the 10,000th entry (i.e. the last one).

This is the result:

As you can see, it's not only considerably slower, it also has a bit of an allocation problem.