It would most likely be better to move all of your XF files into a different folder. That way you don't have to deal with changing your licensed URL later, or needing to update board paths again, rebuilding the system cache, etc.
I would:
Move XF into a subfolder (like "old).
Install IPB in the root. (Don't forget to choose to download the converters when you download the software! It's a checkbox that is sometimes easy to miss.)
Install IPB as normal.
Once complete, login to the ACP and do some basic configuration (site name, etc). I might also create member groups and set their permissions.
Turn the forum offline.
Go to the converter section of the ACP and select new conversion and choose XF. Point it to the database and provide the system paths of the files. (These will be server paths like /home/username/public_html/old/avatars)
Once the converter completes, check over the rest of the settings/permissions! IPB and XFs permissions don't quite match up, so just make sure you don't accidentally expose your moderator forum, etc. 🙂
Once everything looks good, turn the forum back online.
If you ever do a manual upgrade, always make sure to download the converters. It is what will manage the old redirects for you so if someone goes to an old bookmarked URL that it gets mapped to the right IPB URL. If you ever remove the converter app, your site will still work... but all the auto redirects will stop working as there is nothing mapping them anymore.