Step 1: Disable vBSEO from the AdminCP Product Manager (also disable the Sitemap Generator)
Step 2: Install DBSEO as per the instructions in the readme.txt file
Step 3: From the DBSEO Control Panel, choose vBSEO Import
Step 4: If you are using forum URLs without forum IDs, turn on the Legacy Forum URL setting from the General Rewrite Settings
Step 5: Turn on the sitemap generator via the relevant page