1. backup your database of new site first and try this.
2. with
SQLYog, you can easily copy over the
user table to a new database. Try it and do not forget to backup first.
3. This requires that you drop your
user table first, it does not merge instead it would copy over dropping the existing table.
4. This way you'd easily get your 15000 members imported to your new site. For those 2000 members already existing can be restored from the backup you made.