AFAIK (As far as I know),
--
You cannot port directly from (other telco's) prepaid to SIMBA $5 senior plan.
You need to do so in 2 steps:
(1) Port from (other telco's ) prepaid to SIMBA $10 SIM-Only 100GB postpaid plan FIRST.
This is because
for prepaid port-in to SIMBA postpaid --
SIMBA only allows port in to the $10 SIM-Only 100GB postpaid plan.
-- SEE THIS
SIMBA FAQ page.
You pay the first month's $10 for the SIMBA 100GB plan -- at the point of port-in application.
(there are no other additional charges to do prepaid port-in to the the $10 SIMBA 100GB postpaid plan.)
(You can also do prepaid port-in to SIMBA prepaid plan; but since you eventually wish to switch to the SIMBA $5 senior plan, there is no point / purpose to port-in to SIMBA prepaid plan.)
(2) After successfully port-in to SIMBA $10 SIM-Only 100GB postpaid plan,
stay on this $10 SIMBA 100GB plan for at least 6 months.
After 6 months, go to
Orchard Central or Kaki Bukit Service Centres and
request to switch to the SIMBA $5 senior plan.
-- SEE THIS
SIMBA FAQ page.
(Personally, I'm not sure the reason why the requirement to stay for 6 months on the $10 SIMBA plan -- before you're allowed to switch to the $5 senior plan -- you can go ask SIMBA directly.)
@barnabaskoh has already summarised the above for you in his reply:
If you wish to avoid the 6 months delay,
switch your current prepaid to a postpaid plan -- at your current telco FIRST.
(make sure there's is no mobile contract -- when switching from prepaid to a postpaid plan at your current telco)
After switching from prepaid to a postpaid plan -- you
can then port your (now postpaid) plan directly to the SIMBA $5 senior plan.