They need the title, the order record number, the ISSN, and the last payment information. The new vendor will attempt to clean up start dates that have gotten out of whack. They must clearly understand how your invoicing works. III has a 500-line maximum for invoices. When the library receives the pro forma, the staff should carefully check it.
Some libraries go over the pro forma line by line.
Aside from providing the vendor with an accurate list of titles, this is probably the most crucial step in the transition process. In addition to title changes, cessations, and fund code problems, you may discover changes in packages or memberships from what the old vendor supplied. Working out an effective method for handling order records for memberships requires some thought. In some cases vendors may need to do two or even three pro formas to get exactly what the library wants. Ask questions up front.