Memberships can be upgraded or downgraded at anytime by purchasing a new level of Builders Club for the account.
When upgrading or downgrading memberships, Roblox immediately replaces the existing membership benefits with the newly purchased benefits. Roblox also converts any remaining days on the current membership into an equivalent number of days of the new membership type, and adds those days to the amount purchased.
When upgrading, regular Builders Club days are divided by 2 to convert to Turbo Builders Club days. The same principle applies for OBC. The process is similar for downgrading a membership. This ensures that money spent on previous memberships is not lost.