I strongly do NOT recommend mirroring with different versions. The mirror should also be 2008 if the principal is SQL 2008. The setup you described is intended to work only for a short time during the upgrade process.
Have you considered log shipping? Ship with standby (also requires same version) and the database is read only automatically. You could set the restore job to restore after hours only so that there are no disconnects.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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