That is a good idea, i suppose that for the costume don't disappear the best solution (for gameforge profit) is creating some kind of item that when drag should "lock" the costume and when it expires the time it get's red and unable to use but doesn't disappear (some kind of protection shell xD), giving the oportunity to transfer the bonus from that locked costume to the new costume in the future on the NPC, and then the cycle begins and if you don't want that new costume to disappear again need to buy that "lock item" again.

Gameforge more than make money from constant selling of costumes, also benefits and manage to take some of the excessive yang from the servers exactly by the changing costume systems, soo for them to make changes on that, they need to gain on the other hand some advantage, so it gives more utility to the "transfer bonus" item and constant sells from that possible "lock item".

That way you said it saves yang and your bonus set on the costume, but it unbalances the market because nobody buys anymore the change costume items from the general store, for that to be possible it need's to be a considerable choice, or you choose to buy a costume and spend yang on it every time hoping for the bonus set you want, or you buy a costume every time aswell and spend more coins on it to mantain the bonus by "locking the previous costume" and transfering it to the new one when you have the possibility to do soo.
If you don't have that possibility, can sell the costume, and do all the normal things that the costume offers such as transmutation, except the fact that the costume is unable to equip, but it's bonus are preserved forever on them, and the only way you can use them again is by transfering on a new suit.

This is probably one of the only ways that gameforge would agree on that idea you said because "the lack of insertion on item shop" it serves a purpose and it is deliberated to make you spent yang on it in constant cycles, and dc's, and even give you the crazy idea to spend a lot money to get those 1 year azrael suits.

Could make the "lock item" to be available at NPC for really high amount of yang - so you buy costume once and spend yang to get the bonuses you want and then you have a choise - either buy the lock item and transfer them to the next costume or spend yang again on the new costume to get the bonuses you want again from 0. This way there is no extra profit for gameforge but we still have to buy costumes and still got to spend yang so the only thing that changes is that we can now save the bonuses for later in case we cant get a new costume before the current one ends.

Sure that is another way to do that and sometimes the amount of yang spent on a costume is high or very low depending on luck, and that lock item from yang is a fix value... that should work too i guess.