First of all I shall extend a warm welcome to our community! Thank you for sharing this interesting question with us. There is actually no general policy for periodic change of modems, except exchange any kind of device that reaches end of lice cycle. In such cases our customers are being informed well beforehand. Are you currently facing any issues? Or is all OK so far?
However, if you wish to exchange your modem, then simply send me a private message along with your customer no. My colleagues will then check your set-up and - if needed - come up with a proposal for a suitable modem. How does that sound?
Wishing you a nice evening.