There is no definitive answer to your question, since it will totally depend on the complexity of SSIS packages, SSRS reports and SSAS Cubes.

Doing [in-place upgrade][1] is always risky, since if something goes wrong, it would be difficult to rollback. A side-by-side migration is always a good and reliable method, since if something goes wrong, you already have a system up and running that you can point to.

Always, test, test and test your migration and have a fully tested rollback plan. Use [**upgrade advisor**][2] *(this touches DB Engine, SSIS, SSRS and SSAS)* and fully address all the issues raised by it.

 - I have written a thorough answer for [database migration and post restore steps][3]
 - For SSRS migration, I have [written it here][4].
 - Devin Knight talks about [Upgrading Packages to SSIS 2012][5] and msdn has a whitepaper on [5 Tips for a Smooth SSIS Upgrade to SQL Server 2012][6]
 - For SSAS, [this method][7] should work in your case as well.


  [1]: https://dba.stackexchange.com/a/29330/8783
  [2]: https://msdn.microsoft.com/en-us/library/ms144256(v=sql.120).aspx
  [3]: https://dba.stackexchange.com/a/50315/8783
  [4]: https://dba.stackexchange.com/a/68161/8783
  [5]: http://www.sqlservercentral.com/blogs/dknight/2012/05/16/upgrading-packages-to-ssis-2012/
  [6]: https://msdn.microsoft.com/en-us/library/hh667275.aspx
  [7]: http://blogs.technet.com/b/mdegre/archive/2010/03/31/migrating-a-cube-in-sql-server-analysis-services-2005-to-sql-server-analysis-services-2008.aspx