Hi Daisy Spiridopoulos, it would be probably be easier to block updates from form fills for certain fields in the Marketo Admin.
Of course with either approach there are certain cases where data could legitimately change and you will miss the chance to capture that data. However this risk may be less of a concern than the chance of a lazy form fill destroying good data.
An alternative could be to use substitute fields in your form - e.g., populate a field called "Most Recent Phone Number" on the form (that looks identical to a standard phone field) and then copy it over to canonical phone field only if that field is blank.
In this way, the original data is preserved and the most recent form value is captured in a separate field and can be inspected by a marketing person or the sales owner to determine whether it should indeed overwrite the canonical value. A more labor intensive approach to be sure but a way of having your cake and eating it too.