Once a form is created, can I change a field type?

For instance, if a field was created as 'short text' can I change it to 'Number'?

Changing the field type to another field type is not possible. You will have to replace the field instead with the other field. Example, if its the Short Entry field and you want it be the Number field, then you need to add the Number field instead in the form (and just remove Short Entry field if not needed).

However, if you meant instead changing the validation type of field from one type to another, some  fields have Validation properties.

Example the Short text entry (Textbox field) can have text or numerical validation type.

I hope this help. Let us know if you need further assistance.