Yoonho,  I don't think it's possible to give you all the information you need in a blog post/reply.  There are many variables to consider when doing migrations, but a good place to begin is to review the installation and upgrade guides provided by the vendor.

Also - you should be aware the upgrading from ALM QC 12.x to a recent version ( 17 or 24.1 ) cannot be done in 1 step.  You will need to migrate to an intermediate version, and then another migration to the final version.

In my opinion, if your organization is going to go through the effort, it would make sense to also migrate to the latest supported Java / Windows server version on the new server as well. In fact, you may need to based on the ALM system requirements.  That will save you from having to do another migration if your IT department
wants to migrate to newer Java or Windows Server OS in the future.

For knowing the effects of character set change, I'm not sure that many people will know the answer to your question. Perhaps your DBA will be able to answer.
But invariably you should establish a non-production ALM environment to test out the upgrade and go through your established migration validation plan/procedure.