Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,182
I'm pretty sure that the voucher needs to be detached before any changes to the underlying ticket can be made --> voucher will disappear.
If TK cancel the flight or IRROP you, then the replacement flight would probably be in full fare J.