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.
When it comes to destroying a civilization, gas chambers cannot hold a candle to echo chambers.