when you switch to FullText, the 24.9M entries are deleted... the process indicate you how. what i mean by going to 3.7 is that the Search engine inside vB will be faster because the code changed... in 3.6 is have some lacks of static search, making your server to run slower and require more resources... (that's why we upgrade usually, to make it better)
sure you can use the FullText in 3.6