Execute action when the Value is enetered or Updated

Hello All.

1)I have a requirment to to trigger action to   create a follow up document when the value in enetered into a field.

2) And update the follow up document when the Value is changed.

The first part can be acheived by giving condtion execute only when the Value is not null. Not sure how the second one can be acheived if I say execute action only once.

Is there a way to trigger action only when the value is changed?

Thanks

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