This thread has some valuable information.

Elite Plus upgrade voucher

2 things I can't confirm and they seems to be against TK's rules, although just like Turkey, rules get bent all the time

1. Some were able to get TK to release J seats when there's none.
2. Some were able to upgrade 2 segments AAA-IST-BBB with stopovers over 24 hours, when you upgrade AAA-BBB, they're supposed to be the shortest possible connections, check award bookings as examples for your city pairs.