Yep, modifying your form templates can sometimes be hit and miss for backwards compatibility with forms from the old versions.
Drastic data structure changes can break the previous forms.
Your approach is exactly what I have tried in the past and I have been met with varying success/no success. Sometimes I've had to 'pad' the old forms with 'missing' data nodes to allow them to be recognised by the current template.
What kind of error are you receiving when opening the forms? Would you be able to provide the template .xsn and sample 'old form' ?