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So, there's no advantage in switching the doctype (SE bots don't check doctypes), but there could be the disadvantage that you are "invalidating" your pages. If it is 3.2, stick with a 3.2 doctype.
You could argue that, because 3.2 is an older and simpler standard, the bots will find it easier to parse than 4.01.
If you are going to leave the pages as-is, best to leave it with 3.2 - otherwise, be careful if you switch to 4.01 to make sure the page isn't affected when the browser switches to standards-compliance mode.