Sure.
Check from the website: http://www.digi.com.my/whatshot/promotions/dr/index.do
Yes, but I believe you need to go to DiGi Centre for plan conversion. :

Do you mean you are changing in the same service provider, or you porting to DiGi..?

Yes u can. Juz tell them to Port in both your numbers as Prepaid First.
This process will take 1-2 days.
After that tell them to switch 012-222* to become a Postpaid line(Pre to Post).
All prepaid needs to maintain a RM10 in their credit at least to switch to become postpaid.
So total up u'll need to pay is RM50+10.
The proccess for Pre to post will take roughly 3-4hrs.
Make sure when porting take note a few things
-Make sure u dont have any outstanding or contract and the line is active
-Make sure u check very propely with ur current operator if u do previously terminate any sup line before. Make them terminate it properly because sometimes it will hang in their system that eventually hinders the process.
-If u currently have a sup and u intend to terminate it and switch at the same time, i suggest that u convert the other sup line(the one ur gonna terminate) to become prepaid first.
It will make the porting to be alot easier.