Depend on which telco. Some need to go to centre, while some can just call their CS and ask for change. How much data is consider enough for you?

My answer is can. Just heading to the respective telco website and do it.

Digi postpaid can change to Digi prepaid but need do at centre. I suggest you the Celcom Digi Kuning 25 plan. It has unlimited calls and the data is more than enough.
So far I know Maxis and Digi can. Just need to do at respective centre. Old Celcom cannot, need to port out.

From digi postpaid to digi prepaid, I did it at the digi center. It took about 5 mins per line (I had 3 numbers to convert) and no new sim was given.

Digi prepaid has monthly auto-renew plans, just need to topup and make sure your line has enough $.

Not sure about autopay, but I top up RM30 per line on the 1st of every month to be safe.

I see.. so the Celcom Digi Edisi Kuning internet speed will be capped at 100mbps.. but i think if capped to 100mbps, its too slow for 5G..

Ya.. that why i said 100mbps for 5G, its too slow because 5G can hit up to 1GBps.. so if Celcomdigi capped the internet speed to 100mbps, then how to utilise the 5G..

Just like buying bmw car with speed can go up to 250kmj, but then realize it got cut off speed at 140kmj only.. better I stick with my current myvi car.. 140kmj can hit, 200kmj also can hit..

Not really. 4g are build by individual telcos. These individual telcos prioritise profit, so they only build infra in places where there is potential. If one telco already build 4g in that area, others will not prioritise the same area to avoid duplication. Hence some places telco x has a good 4g, telco y has bad reception.
As for 5g, DNB builds it and all telcos are force to use the same infra.

Yes, that current 5g speed is more than enough for many.
DNB is an entity wholly owned by MOF and mandated by gov to be the single neutral party to deploy 5G nationwide.