Data that had the deleted choice item as selected in them will appear as the values they had without a label to the value.
The most recent label i.e. new one would be the one applied for all data that has the value corresponding to the label.
These being a representation of the data, which are dependent on values and labels, the most recent mapping based on the last deployed form will take precedence as evidenced by what @Kal_Lam had just shown. Omitted (deleted) labels will not appear except for their values, Changed labels will take the form of the new labels.
Summary: Changing labels and values within select questions should be done carefully since the potential of value and label mix-ups are quite high.
On the last point
If by constraints, you meant restrictions on what letters can or cannot be used on the choice names. Beyond the instructions of the general XLS forms, we have not tested what restrictions should or should not be used. However, from the screenshot below, when one has a space on a select_one and they convert it to select_mutiple, there is bound to be an issue.
Regards,
Stephane
