I do not think that it is possible to convert from postpaid to prepaid 
https://forum.lowyat.net/topic/1501043/+2040

Not sure if it fits your usecase, but at rm5 per month(umobile -> umobile) or rm8 per month(umobile -> other operators), you can get either 100 minutes for the former and 80 minutes for the latter

Source: http://www.u.com.my/postpaid-packages/postpaid-add-on-pack

It is the latter. You'll be subscribed for it for every month unless that you do not have the enough of amount of credit for your data plan.

fyi, there'll be a notification to remind you to reload in the case that you don't remember when will it expire.

Why do you prefer postpaid btw? The billing statement? Call history? More quota(extra 250mb for those who pay via debit card automatically for each of the month) or what not?

What is your usage(number of calls, frequency of calls, number of smses, monthly data usage) right now? Let me decipher it and see whether its suitable for you to go for prepaid or not 

okay, then give me an approximate value of your calls(fees spent on it) and smses(fees spent on it), just an approximate value, no need to be exact 

well, providing the community with a better choice is always good, no? 

Summoning digilife 
If you are willing to compromise for your data plan quota, then UMI28 is 

RM20 of calls = 20 / 0.20 = 100 minutes(not exactly correct but you'll get the rough idea)

If you plan to:
1. Keep existing plan(if you want to keep your number)
Apply for the Postpaid Voice Plus Talk Plus Addon pack for RM8 for a 80 minutes call to other operators per month
http://www.u.com.my/postpaid-packages/postpaid-add-on-pack

then, for the remaining 20 minutes not covered by the addon pack:
20 * 0.2 = RM4

Total Call Usage: RM 8 + RM 4 = 12

Comparison: 12 vs 20 = ? 

2. Change the plan
Apply for UMI28. It has 30 free sms to all operators, 100 free minutes of calls to all operators and 500mb of data quota.

so,

Based on your usage, your usage is 100 minute - 100 minute of the free calls = 0(not exactly 0, but even if so, the prepaid's call rate is cheaper than postpaid so it is still cheaper  )

Comparison: Call usage reduced by a lot, but data plan only 500mb

An idea of thought:
Since a direct conversion of plan from postpaid to prepaid is not possible, then what if you mnp it to another telco then mnp it back to umobile? 

anyway, Summoning digilife 

As a student, I believe that you would want to save as much as money that you can though, so that you can use it elsewhere 

I think that you're better off pming him instead 

and, just saying, after I've asked U Mobile's customer service, they said that you can change from postpaid to prepaid directly(verified by another member in the post above as well), so, what are you waiting for? 

yah sorry I missed that sms part, i meant voice calls only

200 sms is unnecessary these days with all the whatsapp and we chat spamming

would love a balance of voice calls added

data speed is the same, just call rate if different , postpaid with add on package is cheaper but no free call to umobile numbers, UMI28 still entitled 1 day 1 hour free call to umobile numbers, UMI28 booster is 1gb 3 days, U28 booster is 1.5gb 3 days, u28 charged 6% tax, prepaid not yet haha

any others ?

After finish 50 min free call to all operator, you still entitled everyday 1 hour free call to umobile numbers 12am-7pm 

no more umobile for me. hahaha so happy and more freedom.