Well, what previous person suggested to you is by getting Prepaid UMobile simpack (super cheap already) at nearest 7-11 store and have a test drive for 1 day. If you're happy then straight up MNP your previous number. Or if it's no good in your area and other places you frequented, then you can stay with your current provider.

By doing the above, in case you're unhappy with UMobile service, you won't risk losing some perks on your current provider (eg: Maxis ONE Club, telco points, etc). IINM, there's no such thing as 'grace period' or 'opt-in period' or even 'tie-in period'. Once you MNP, you can only go back to your telco by doing another MNP process. Means that you gonna lose all your previous loyalty benefits (if you got any).

Mind you, MNP do take time (easily 2 days). And you gonna pay for registration fee, advance fee, bla bla bla in case you wanna revert your previous telco on postpaid, just like when you're starting fresh before. Plus, with some telcos having adjustable call rate based on usage (in case you're currently on celcom), your bill will be prorated but then you gonna start back with higher rate once your billing cycle starts. And if you intend to revert to prepaid, some telco can only proceed that at centers (Celcom).

Trust me, save all the hassles by trying UMobile on prepaid card first, just go buy any number. Been there, done that. Easily saves lots of headaches, money, and petrol in case you're not satisfied.

Pay for it la. Always want free stuff, so lol.