The best way to make it happen is to buy up to J, and your change fee will be waived. The second best way to make it happen is to buy up to Y/B/M and use miles or SWUs, and you may still have to pay the change fee (make sure Z inventory is available first).

If neither of the above appeals to you, you may be lucky enough that economy will be oversold by enough seats that all the non-revs, all the DMs and all the PMs on higher fares than you get upgraded and they still need additional op-ups. In other words, be prepared to sit in economy.