It's very easy to exchange the card..when you arrive at Changi Airport Arrival, there is a Changi Recommends Booth, just give the barcode from the traveloka application to the officer, so you can get an ez link card..then go down to immigration, scan your passport to the machine, take a photo of your face, scan your thumb/big finger..pass immigration, get your luggage, go straight down to the MRT, top up your ez link balance at the machine because the balance you bought from traveloka is only SGD 5..Minimum top up at the machine is SGD 10, you can pay using cash, put it in the machine or use a credit card...I filled in 10 SGD, there's still about 3 SGD left..actually you can use a mastercard to ride the MRT in Singapore, but it seems more expensive. because there is an admin fee. like if we buy an MRT ticket in Jakarta, which is 1, there is an admin fee so it's more expensive