There is no TN for that.
With LTO7, you can keep the LTO5 tapes, because the LTO7 drives will be able to read the LTO5 media.
The only thing that you need to do before trying to restore from LTO5 tape, is to change the density of one or more LTO7 drives to match the density of LTO5 media.
e.g. if LTO5 media is type HCART2 in NBU and LTO7 drives are HCART (these are NBU defaults), you need to change the density of LTO7 to HCART2 before the restore.
Tape drives can read 2 generations back, but in NBU the densities must match.
(There are multiple forum discussions here about restoring from old tapes.)
If you want to acquire LTO8 tapes, you will probably need to duplicate all images on LTO5 to LTO8, or else keep one LTO5 drive for restores.
You need to decide what to do here - to duplicate, you will need to keep one or more LTO5 drives for the read portion of the duplication. The duplications will be resource and time consuming - only you know if you have that luxury.
There are also many forum discussions about this....
