How to ensure that transactions are attached to dunning and predelinquency letters?

Summary:

We would like to know how to make sure that all transactions including the ones before changing the setup are attached to dunning and predelinquency letters.

Content (please ensure you mask any confidential information):

In the AR_FEATURES lookup type we've created a lookup code AR_STORE_PRINTED_TRANSACTIONS to ensure that transaction will be attached in future. And in the collections preferences we've set "Maximum number of transactions to send with dunning letters" to a value of 500.

We're now unsure how to make sure that historical transactions without attached pdf-version of the printout are attached to the dunning and predelinquency letters.