I recently switched from MyBB to ElkArte and it went smooth for me. Thought of sharing the steps here, so that it will be useful for others.If you have the backup of MyBB database:
1) Install a fresh copy of ElkArte in your root directory either by uploading the files using FTP client or using Softaculous.
2) Once done, do the same for MyBB by installing it in any folder of your liking.
ex: http://yoursite.com (where ElkArte is installed) and http://yoursite.com/mybb/ (where your MyBB is installed)
3) Now go to phpAdmin from cPanel and choose the MyBB database file.
4) Now click Import tab and upload your backup MyBB database.
5) Once done, close the phpAdmin and go to cPanel.
6) Now is the important part where you give rights to users to access both database.
7) Click SQL database in cPanel and go to the section “Add User to Database” and add MyBB user to ElkArte database and ElkArte user to MyBB database.(Link-5733)
8.) Now download a copy of the OpenImporter from ElkArte. [https://openimporter.github.io/openimporter/]
9) Extract the files to a folder and upload the openimporter folder to your root directory.
10) After that run the openimporter by using the following link [http://yoursite.com/openimporter/import.php]
11) The importer will run and in the page choose your MyBB version (1.6 or 1.8.)
12) After that, in the next page in "Path to destination” let the path be “/public_html” and for “Path to MyBB 1.8" be “/public_html/mybb. If the path are right both will turn green and you are good to go.
13) Now enter the database password of ElkArte (if you don’t remember, get the password from settings.php)
14) Click the Advanced options to choose what you like to import. After that click the “Continue” button.
If you have set everything right, the import of MyBB to ElkArte will take place and it will be done depending upon the database size (I believe xD)
These are the steps I followed and it worked perfectly, if you have queries or anything needs to be modified kindly let me know.
Thank you very much for sharing your experience and a step-by-step guide! :D