These are:

1. account receivables and payables files: make sure all bank codes are converted to up-to-date BICs (Bank Identity Codes) and the bank account numbers are converted to up-to-date IBANs (International Bank Account Number). You can do this yourself or use a third party processor to this. You cannot rely on your bank to do this because in some countries, the conversion is not automatic as there are local variations, they jsut won’t recognise the number. At the start of the SEPA migrations in 2009, it took 6-9 months, but now processors, such as Experian, typically take 1-2 months to carryout the conversion. If you can achieve this, most banks will be able to create the new XML payment formatted payment messages from your old payment files and make your payments.
2. payroll payments: need to be able to be made via the new SEPA Credit Transfer XML formats.

Implementing these two things will enable you to survive. To exploit the brand-new business opportunities from SEPA - rationalising the number of banks and accounts, introducing new business transaction processes using the extra data attached to each payment, etc. - will take much more, but, at least if you do this, you will survive.