Upgrade to membership for higher levels of access?

If a member wishes to upgrade to a higher level of membership, how can that be done?

In other words, if we create access levels of Bronze ($115/year) and Silver ($134.95) and someone buys a Bronze level, but midway through the year wants to upgrade to Gold level for the additional cost, how can we accomplish that?

We currently have a membership that has access to some functions, but not all. Some members will want to have a higher level of access, which we would sell for $19.95 more than the regular level.

Thanks for your help as always!