Just to follow-up on the comments from Applejuiced concerning FAN discounts. No impact to your primary line when you use it on a secondary line. Just no discounts will apply on the line that the original iPhone is registered on.If I do this trick (not on the primary line), would employer discount on the family plan be removed even if I go back to 4S in couple of days? That wouldn't be worth it for me then.
Please note that if you use the original iPhone loophole on your primary line, you will lose any FAN discount on that line as long as the IMEI of the original iPhone is registered on that line.
Your FAN discount will be restored once you update your account to remove the original iPhone's IMEI and replace it with another IMEI.
Dave