You need to call CO and ask for your specific ticket's rules. If there is no language in the fare rules forbidding what you want to do (there usually isn't), then the generic CO procedure is to have you pay only one change fee here (when you buy the first $333 ticket, at that point you pay the change fee). You then get a $817 voucher which can be used as you like, with a new 1-year deadline from the date of voucher issue. No more fees at that point.

(Since this is an international ticket, are you sure the change fee is $150, by the way? I'd expect $250 for a discount international ticket these days.)