How to "Replace"

[ohrenkino]

You can only replace what is already there.
So in fact, I think you want to copy one tag field to another.
There is a post in the FAQs for this task:

Actions and batch-operations

How to copy one field value to another field? Example how to copy the content of the ARTIST field to the ALBUMARTIST field: Create a new action in an existing action group or make a new action group. Action type: Format value Field: ALBUMARTIST Format string: %artist% Note: If you want to copy all possible values of a field (in case you have multiple artist fields), the internal scripting function $meta_sep can be used in the format string, e.g., $meta_sep(artist,\\\\)

For the conditional check use as
Format string: `$if2(%originalyear%,%date%)
Afterwards you have to remove the field DATE either in the extended tags dialogue or with an action (as part of an action group).