I changed mine at centre. Not sure it can be done over the phone CS. Give IC, make outstanding payment, then wait for few hours, no sim changed, once changed to prepaid, it reset to RM5 with 10days (if i remember correcly) validity

Its depends on your usage.

Prepaid 1GB with free FB.
If u surf FB a lot, prepaid with umi30 will be better than u28 postpaid. Provided the umi30 promo last forever.

Attach also the umi25 which price nearly the same as 28. But in term of value, u may justify urself.

if you are focus on making call then the ans is no.
as prepaid plan offer 12sen/30sec and 30mins free call on umi30. while u28 offer 10sen/30sec and 50mins free call.
if you are focus more on data then the answer is yes.
as prepaid plan offer unlimited app onz and 7.5+2.5GB data
while u28 only offer 3+3GB data
yes.. the basic internet only count if you have no subscribe to any umi plan