You say "old" -- if the SIM has been deactivated (e.g. you can't use your G2 with said SIM), then, no. You'll definitely need a new SIM.
If the SIM is the same size and will fit in the new device, then "generally yes" -- although, T-Mobile recommends upgrading your SIM every few years since there's updated information on tower connectivity and other features (WiFi Calling being one example) in newer SIMs.
Thanks for the information Simple....- very helpful. The sim in the old flip phone is still activated and in use but 2G only intermittently functional in my area depending on exact location. Good advice about need for updating sim.