No you cannot lose your data when changing the forms. Ideally the system allows you to get all the data from all the versions of your forms when downloading it. So if your first form had fewer variables/questions/columns then the data collected on that will only have responses for these variables with the data collected on the later form definition having data collected based on that form definition.
It should however be very important to maintain the response choices as they were in various definitions e.g. if Yes=2 and No=1 in one question, you should not change it to something like Yes=1 and No=0 since data will have the values meaning totally different things.