cognos has a nice little tool called the lifecycle manager ,which helps you migrate from older cognos versions, checkin your report specifications and so on.
Coming from Cognos 8 (esp. 8.4) is not that big of a step in my opinion. Just keep in mind that also the newest Cognos Version 10.2 has some changes to 10.1.1. that might have an impact on your migration as well.
For example, i faced the issue that coming from 10.1.1 to 10.2 some datatypes in framework manager (Oracle DB 10g behind) changed and using keys with different datatypes does not work properly anymore in 10.2 (search in the search box in the upper right cornoer for more details).
gl and cheerz