Subform entries compare existing input values to see if a user has changed an input value

Hi

Hoping this is simple, think i could do this within a normal form as you have the "old entries" feature

Problem

Subform where a user enters 4 entries

each row is
date1, date2, amount, qty, (confirmed blank text)
Upon validate success, i write yes in each subform row "confirmed text field"

Ok

User comes along and edits any of the rows.

if they change date1 or date2 or both i need to know the existing date values so i can can cancel the existing delivery dates

Next
if they change dates i also need to make confirmed = no.
This does not really work as for a date field on user input will get triggered even is the date is the same, it is user input i guess no user update

My only idea at the moment is to duplicate all the fields date1 and hidden date1, upon first entry set them up to be equal and from then on compare them looking for changes but i can icebergs ahead with this idea

an suggestions would be welcome

thanks