Hmmm....That will need to implement some logic... You can try something like below.
In view initialization, create a static variable gv_pop_up with the current value of this field. (First clear it and assign the current value).
In event EH_ONSHOW_POP_UP, write the following code before the call of pop up.
lr_ddlb ?= typed_context->context_node_name ->collection_wrapper->get_current().
lv_value = lr_ddlb->get_property_as_string( iv_attr_name = 'YOUR_FIELD_NAME' ).
Now you have the new value selected in lv_value.
Compare the old value with new value (lv_value and gv_pop_up) and then if user presses NO, assign the old value to the drop down. I hope you know how to set the collection. Also, if user presses yes, don't forget to update the static variable with the new value so that it will compare next time with this value.
Thanks,
Faisal