Dave Noble is correct, the rule refers to an increase in continents [and]/[or] extra flight segments (i.e. when buying additional segments for being over a continent's limit). There is no mention of taxes.
So if you change from a DONE5 to a DONE4 or go from 5 EUR segments (for which the additional segment collection* has peen paid) to 4 EUR segments there will be no refund. e.g. you will
not get the USD400* refunded if you bought an additional segment on a DONEx.
Interestingly, the
rules as published on the oneworld web site gives an example to make this clearer:
Code:
16. VOLUNTARY CHANGES/REROUTING/PENALTIES
(a) Rebooking/Rerouting
(1) Prior to departure
...
(2) After Departure
(a) Date/Time changes permitted at no charge. ...
(b) If the rerouting results in an increase to the number of continents
or extra flight segments previously charged, the ticket shall be
recalculated. Tickets may be reissued to a higher fare, eg. from A
–ONE3 type to A -ONE4 type fare and the increase in fare will be
collected. When flight segments are added above the free allowance,
those charges will be collected. When the rerouting results in
a reduction to the number of continents or extra flight
segments previously charged, no refund will apply.
*
Additional segments are USD400.00 / AUD500.00 / GBP200.00 / HKD3200 / EUR300 for DONEx's