So when I was running 1.6 I made the ACP directory be a custom name to help mitigate hacking attempts. But since I've upgraded it kept both ACP directories.
Would it cause any problems if I just delete the old directory?
Did you rename the admin directory on the 1.8 download before uploading them to the server?
If /admin is your new administrator directory, there will be no harm in deleting the old one, although you may want to migrate any additional modules added by plugins that you may have installed in your MyBB 1.6 forum.
Well it would if they have different names. You would need to delete the old one, upload the new one, and rename that to what you had called it before.
Thanks for the answers, I managed to fix this by just deleting the old one and renaming the new one.