You can always upgrade from a Business name to a Limited Liability company at anytime of your choosing. However, this may take more time and money than registering a new company depending on the type of upgrade you are doing.
There are two types of upgrade or conversion: Direct and indirect.
Indirect conversion involves adding one or two words to the already existing business name. In this case the CAC see the upgrade as registering a new company. The cost for this is the same as a new company Incorporation.
Direct conversion means you use the same name and only add "LTD" at the end. This will require closing down the business name and then applying for the conversion. This may cost double the time and fees of registering a new company.