Hello Thorsten
This is a very generic question. Are you talking about a migration from a previous version of SAP ERP to Suite on HANA, or S/4 HANA?
In general, if you use OpenSQL in your ABAP reports, there will be no need to modify it if you migrate your ERP to run on HANA DB. But you can rewrite them if you need a better performance and want to use the capabilities of HANA to improve it.
If you use Native SQL for some report, of course there will be some adjustments due to specific DB syntax.
I hope it can help you, but you can specify if you have other doubts.
Regards
Luis Becker