If a member is within a membership level that is set to expire (the Length of Subscription has been set for the membership level) then the member cannot renew their membership before the membership level expires.
This is because they are still an active member until the subscription runs out.
Once the membership level expires, the member can then use the Existing Member login option on the registration form to re-register.
Note that if there is a Length of Subscription set for the membership level then the Un-Cancel on Re-registration option should be turned on for that membership level if members are allowed to re-join the membership.
This allows members to be able to re-register once their membership expires.