My mom has a Singtel prepaid SIM card that is quite old and still using the full size sim format.
Can SingTel replace the SIM card for a nano SIM?
There is no free Prepaid SIM replacement for all Telco which require SIM card charges.
Some Telco required a valid existing Prepaid SIM in order to have the replacement and may include transfer of wallet credit, if any.
Either you go for the replacement with cost or most phone shops able to cut the SIM card to nano size for $3 cost.
Alternately
The other method is to use finish the wallet credit and do porting to another Telco Starhub/M1 and the Prepaid SIM with pre-cut sizes.
This way you could enjoy free replacement Prepaid SIM as paying for the wallet credit and some do come with free data usage.