Question Threads 2.1.0 was rewritten from scratch. It is compatible with previous addon version but only works with XF 2.1 and above! If you are upgrading from version 2.0.2 read the instructions below!
Upgrading from Question Threads 2.0.2
Make a backup of database!
Disable (not delete!) old Question Threads 2.0.2 addon
Add $config['CMTV_QT']['upgrade_install'] = true; line in src/config.php file
Install new Question Threads 2.1.0 addon
Delete old Question Threads 2.0.2 addon
Delete added line in src/config.php file
Check addon permissions
New features
Now marking question as "Solved" and selecting best answer creates a news feed entry which is displayed in "Latest activity"
Fully reorganized style properties
Changes
Improved addon stability, better code quality
Question and best answer marks are now correctly displaing everywhere
XF 2.1 and PHP 7+ are required now to install the addon
Question and best answer marks are displaying correctly in tags search pages
Users with zero best answers will not appear in "Most best answers" list (@katsulynx)
Fixed some bugs with threads merging (@Recep Baltaş)
Now you can't merge two questions with best answers.
Question Threads properties are fully depend on target thread. The result of merging a solved question with best answer to a simple thread will be a simple thread.
Backup your forum (both database and files) Seriously!
Go to your forum root and remove these three folders: src/addons/QuestionThreads, js/questionthreads and styles/fonts/questionthreads
Download addon version 2.0.0 and move contents from upload folder to your forum root
In admin panel go to "Addons" section and upgrade the addon
After updating click on cog at the right and select "Rebuild"
After rebuilding return to ACP main page. You will see an yellow message at the top saying there is a job to complete. Click the link in that message and it will automatically recalculate best answers for all users.
Fixed an issue where the best answer icon is not changing in style settings @bolibick
Fixed an issue where the best answer icon would stay "diamond" after changing it in style settings @bolibick
Fixed an issue where an option to convert thread to question was appearing when editing every post in that thread (not only when editing first post)
How to upgrade?
Download an addon archive and unpack it somewhere
Open upload folder and move all folders there to your forum root directory (with overwriting if needed)
In admin panel go to "Add-ons" section and upgrade "Question Threads" addon
Important! If you can't upgrade an addon and seeing "Version mismatch" message than you need to execute one simple SQL query (for example, it can be done via phpMyAdmin):
Added an option to mark already created ordinary thread as question when editing first post in the question
Fixed bugs
First two bugs were found by @Mr Lucky. They are fixed now!
Fixed an issue where best answer icon was replaced with horribly looking icon name text
Fixed an error related to emailing system that appears after uninstalling addon
Fixed another issue with questions converting to ordinary threads when editing first post in the question
Fixed an issue where "Is question" checkbox was or was not selected incorrectly
Fixed an annoing issue where saving forum settings with "This forum is for questions only" enabled would always start redundant "Converting threads to questions" process every time
How to upgrade?
Download an addon archive and unpack it somewhere
Open upload folder and move all folders there to your forum root directory (with overwriting if needed)
In admin panel go to "Add-ons" section and upgrade "Question Threads" addon
(Not likely to happen) If best answer icon disappeared after upgrading then go to "Admin CP -> Appearence -> Style properties -> Question threads colors" and select any best answer icon. Clear the cache (usually Ctrl + F5) of the thread list page.