No, as long as the costume itself is originated from TP content it will be converted to voucher.
Is there any restriction regarding the change ? i’ve read that if you buy a raid weapon through trade or market you’re not going to be able to change it on the event right ?
I don’t know the specifics, however i believe that as long as you own it prior the update you should be able to exchange it
Is re-leaning attributes going to be time-gated?
I believe so but i can’t confirm that.