Modem is driver using AT commands that are divided into several groups: common AT commands, GSM AT (GSM 07.07) and SMS AT commands (GSM 07.05). Even the specification exists, implementation differs, especially sense of particular command parameters. Another difference is utilizing of the PDU and text format of SMS.
Since both (PDU and text format) are supported it's reasonable chance then new modem will behavior the same as an old one. It's to be checked.