That works, I was about to do it last year. Except you would then get a new voucher for 460 EUR valid 1 year from the date of the flight booking, and another 40 EUR voucher valid 1 year for the date of the cancellation of the 40 EUR refundable flight. To keep only one unique voucher, you need to buy a 500 EUR+ refundable ticket, cancel it and get another 500 EUR voucher (and cash refund for the part over 500 EUR).

In my case, I discussed this with the platinum service line and they told me not to bother with this strategy and that they will take care of that and issue me new vouchers after the old ones have expired, and indeed, they did, no hassle and great service!