When changing your back office system an important consideration will be what to do with your existing client data.
Whether this is held in another commercial back office system, a "home built" system or whether you use spreadsheets currently, it should be possible to transfer the data to your new system.
Often the data transfer process is routine and can deliver satisfactory results affordably. What's more, the data that you hold can be "cleansed" to ensure that any missing data is identified and populated.
Typically conversion costs vary due to a number of factors; such as the system from which data is to be transferred; the extent to which the system has been used; the volume of data involved and the requirements for data cleansing.