Hello,
You can use old DP backup tapes with DP 8.0, either by them being part of the Cell when DP was upgraded to 8.0 or by importing the tapes after DP was 8.0.
However there are LTO hardware compatibility limitations. I don't think an LTO 6 drive can read LTO 3 tapes. If you think you might want to do this and you are not sure, either take a look at LTO documents online, or ask the question in the tape library forum.
If you are keeping an LTO 3 drive in your DP 8 cell then you should be fine.
All the best, Stewart