Regarding married segments -

I tried to use my E+ vouchers for JFK-IST-XXX and upgrade only JFK-IST.

However, at IST the lady said only TK4 is available for upgrades (despite looking at flightstats.com and JFK-IST J is widely available on all 3 flights [4,2,12]). Rechecking flight availability using JFK-XXX and yes, only TK4 was available for TK.

Question is, if I purchase just JFK-IST now, will I be able to use the vouchers? I'll just purchase IST-XXX seperately later. I'll try this later next month and report back.