You should get the OWE price that was applicable when the ticket was originally issued, i.e. the pre-April 15 price and rules should apply if you downgrade to a OWE5.

Changes are free before departure but do not change the first segment or you will lose the pre-April 15 pricing and rules.

Your TA should have given you an open dated ticket (except for the first segment). Let him do the work of changing the reservation dates. There shouldn't be a problem doing so. Note that route changes are chargeable but date/time changes are free.

Given your description of what your TA has done, it seems he may not be totally familiar with OWE ticketing and rules. Watch him closely.

The Kiwi dollar is very strong now so I gather that you are dealing in another currency.