You cannot downgrade plan..have to port out..then port in back..

Suggent port to umibile than back to CUP35..

Afaik, Celcom no longer offer First plan?

If you want to maintain Celcom line, best option is to port out then port in back to Xp Lite or Celcom Prepaid or port out to other MVNO like Yoodo.

why not get a temporary prepaid line to fill in the gap? until your current line gets into new bill cycle.
at least you don't unnecessarily burn, buying only to die in a few day.

I am not well-informed of current prepaid offerings.
So can't really advise you on the selection.

What I can think of right now
you can either stick to celcom prepaid since that already work well for you, or, ask your housemates if they are using another telco and then you buy that other telco as a comparative experiment if it's better than celcom.
of course, just get a mainsteam digi celcom maxis umobile line from 7-eleven, don't need to waste time to hunt for exotics like yoodo, red, etc, since it's just fill-in-the-gap immediately.
oh, i think 7-eleven got cut off time, after 8pmnor 9pm cannot buy new prepaid line aince they can't register you immediately after that.