Digi prepaid?  No limit on monthly unused data rollover.  Can accumulate up to 100GB.  For postpaid, depends on plan.

This is the plan available to some of us. Confirmed Digi upgraded it with rollover now. Just renewed mine today.
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Yes the CS told me it is a monthly plan, this is my 2nd month. It is like the rest of Digi prepaid plans, auto renewal with on time renewal extra 2GB for customers over3 months with Digi. The 7GB (2+5GB) only the unused 5 GB is rollover. Total GB is 7GB (2+5) + 2GB = 9GB plus all other prepaid freebies.

prepaid but auto renew on every 28th with your prepaid credit

I've contacted Digi regarding the Box of Surprise RM28 plan, that comes with 7GB+unlimited calls+1GB/day 1-7pm.
Although the "All usage internet promo (2GB)" will be consumed first, but it cannot be rollover.
Only the remaining unutilised Internet Quota (5GB) will be rollover.

Hope the answer of this attractive plan would satisfies your curiosity. 

Surprised Digi is offering me this in the BoS:



This will lower their ARPU (I was on biGBonus 30) but they did offer this to me so I'm taking it.

I lose 30-odd GB rollover data from my previous biGBonus 30 but I'm saving in the long term.  I gain unlimited calls although I hardly make calls but I'm not complaining!

Lucky I didn't call CS, LOL. Anyway this is what I saw. I just went with it.

Let your main line idling and don't subscribe any package, wait for BOS

What do you mean by "transfer"?  AFAIK you buy rewards associated by each number under your name; I don't think you can transfer rewards between accounts/lines.