The guys will have to answer this but here is the issue - what page are the likes attached to? If it's a general forum index page, e.g. mysite.com/forum/ or /index.php, it would see possible not to lose those likes from the page. However, if you're talking about individual pages, you'd have to make sure that the IPB URLs are maintained.
I don't know how you set them up. I love IPB's product but I think that the URL structure, unless you really know what you're doing, can be mixed up pretty badly, especially with regard to setting up the CMS. My gut feeling is that if you're here the Likes aren't going to be missed and you can always get some back -- and probably get even more using the software. It's very contagious.