I have update two blogs to 126.96.36.199 without any spam problems. One is a new WP blog, the other has been online for about a year and has spam prevention in place. When I moved from b2 to WP, I didn't get any spam for awhile but it eventually started and even though all my comments were moderated, the spammers hit me anyway, often looking for my b2 files.
The upgrade has nothing to do with you getting hit now. It just took awhile for the spammers to update their spam scripts with your blog's WP files. If you are still using the same host and domain, you should be able to see your logs and they will tell you if the spammers were hitting your site with your old blog scripts info. That's how I know they were trying to hit me with my old b2 prefixes.