Where would it be possible to have the expiry date changed to dropdown?
In an ideal world it would have months and then year for the other one. The year part would ideally take the current year plus 6 and populate that, so at the start of 2019 it would lose the 2018 automatically.
Just finding people getting that part wrong.
Sure, you can use an override for the credit card template and change the expiry to be dropdowns. Be sure to use an override for the cc.tpl template rather than modifying distributed files.
If your payment method uses a template other than cc.tpl, then you can simply override that template too. Feel free to contact me if you need some help.
Template is located here
design/themes/THEME/templates/views/orders/components/payments/cc.tpl
Template is located here
design/themes/THEME/templates/views/orders/components/payments/cc.tpl
but you should use an override so your changes are not overwritten on upgrade. Never modify core files if you don't have to.