well, first, the reports are actually stored as binary in tthe ReportServer database, so there's no direct requriement to extract out the RDLs.....but it can still be done.
to migrate your reports, first you need to save the encryption key from the current SSRS reporting instance. if the current server is different, or you rename the server, you have to have that key to restore to get your instance back..
then you just create your server, or do your in place upgrade.
If you create a new server, you need to import the encryption key into reporting services, restore the database on the new server, and finally tell SSRS what database to use.
if you want to extract out the RDL's, there are a number of SSIS packages and powershell scripts out there that can extract the binary from the database, and save them to local disk as rdl and whatever the datasource extensions and all that is.
just google "powershell extract SSRS" or "SSIS extract SSRS" and find one you are comfortable with.
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