I would leave out the removal of all tags and update only those that are incorrect.
If you still want to do it, use an action of the type "Remove fields except" and leave the field %dummy% as the last one.
An import of data from filenames can be run in an action with an action of the type "Guess value" - the syntax is the same as in the function Convert>Filename-Tag which means it works only for files with the same pattern in the filename.
To set a certain field, e.g. COMMENT, to a certain value, use an action of the type "Format value".
Actions can be grouped in an action group.
To see more about actions, see the FAQs