How to apply expr to column in-place?

[mdmcconnell]

I can see in the examples how to apply an expr to a new column using AddColumn with an expr option. Is it possible to apply an expr to an existing column? How?

Also, datatable depends on a forked version of expr with some non-obvious differences (such as: ~ is the concatenation operator for datatable). Is that by intent, or because new versions of expr have superseded the one used by datatable? Is there any thought of bringing the expr version back to main branch?