easyJet b do something similar. They waived flight change admin fees, due to Covid, but the new flight will be charged considerably more than if you purchased it directly. They simply add the admin fee they would have charged to the price of the first flight.
I changed two flights to two new ones. At the same time I bought two additional identical flights. The first leg of the changed flights was over £130 more expensive than the additional. easyJet of course deny any wrong doing saying they don't have a published price.