Returning to stock rom from CM 12.1 to Android 4.2.2

Hello

I am currently running CM 12.1 on my SGS3 i747m.
I want to return back to stock rom, the current version offered by Rogers is Kit Kat 4.2.2.

Is this considered a downgrade? (from CM 12..1 to Android 4.2.2) Will I brick my device?
Is returning back to stock rom the only way I can network unlock my phone?

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