Generally, the voicemail upgrade will not change telephone forwarding configurations. The typical scenario is an office telephone which has been forwarded to a peer's telephone. Another likely scenario is to forward an office telephone to a cell phone. None of these situations will be affected by the voicemail upgrade.
If your telephone is part of a Voicemail Tree (auto-attendant) it is possible this forwarding will not work during the upgrade maintenance windows. ITS staff have contacted the owners of these voicemail trees to find alternative solutions during the upgrade.