Does changing username affect progress?
No, you won't. Players are tracked with a unique ID, instead of your username as of Minecraft 1.7.10. You can only change your username every 30 days, and if someone changes their username, you cannot change it to their username for 37 days. As for servers, you'll have to ask the server owner, but you usually don't lose progress. https://help.mojang.com/customer/portal/articles/928638-minecraft-usernames