Generally speaking, I would say apply an R9.1 upgrade license, upgrade to R7.0.36, then to R8.0.66 (this should generate a Virtual Essential Edition License), then to either R8.1.95 or any R9.0 build and from there you can go to R9.1
As other's have said, there are some cards and features no longer supported in 9.1, so would be best speaking to your business partner/distributor first before upgrading.