The statement you start with ("with the same firmware") still holds.
6.5.4.14.is available for IAP-305. If you can't access it for some reason, that should be fixed first.
Also, IAP-205 is not supported in software versions higher than 6.5.x.y (the platform is "parked" on 6.5), so it will not be possible to upgrade a cluster that contains both IAP-205 and IAP-305 to an 8.x release.
And yes, you may have to downgrade.