HI Reynir,
Upgrade to release 7.4 is not limited to running on the HANA database. You can run on any (supported) database. I am not 100% sure if this will be the case for release 7.5. I get the impression that this is really a HANA release.
Although there are certainly new features for release 7.4 w/o HANA, the real benefits can be achieved on HANA. So you will have to think well about the timing for an upgrade: together with a migration to HANA or upgrade using the existing database. Also consider release 7.5 which is now in ramp-up.
Best regards,
Sander