Larivat
The only thing you can guarantee will not affect previous data is conditional formatting, all other operations will change at least the user operation at most will change the data.
Validation will prevent previous forms from submitting unless done carefully. You cannot change the name of fields, data connections etc.
To help you you have to publish the amended form under a different name in the shared directory OR go into 'tools', 'Form Options' select 'Advanced' and change the 'On version upgrade' drop-down box to "Do nothing, exisiting forms may not work properly".
Hope this helps
Adam