That seems fair so long as someone would have the chance to do the same trial as their earlier stages in order to get their upgrades to be on par. For instance if it was, kill a Delve Boss 5 times, and I did, my 95 Excalibur would be upgraded. Now if I collect 5 Marrows I can turn it in and my Excalibur will be as strong as a level 99 Excalibur became once upgraded. I can see that as being perfectly fine, and not a bad idea in the least. No one who did get their weapon to 99 is losing out and no one who didn't is left without the option to do so after the fact.