In theory, connection poolers are supposed to be plug-compatible components that expose the DataSource interface, so just upgrading Tomcat isn't supposed to be an issue. And, as far as I'm aware, there's no reason a pooler that worked with Tomcat 5.5 wouldn't work with Tomcat 6. Tomcat 5.0 is less certain, but that's because of the JVM requirements changes.
A bigger problem is that I'm not sure how viable Primrose is these days. Their website is broken at the moment. If they're an open-source product, it should be possible to persuade it to work under newer versions of Tomcat. If it's proprietary, you may be out of luck.
An IDE is no substitute for an Intelligent Developer.