What will happen to my database when I upgrade?

I’m not using WordPress yet. Rather, I have a series of websites I designed myself. They’re essentially one big CMS, with static pages on MySite2, MySite3, etc. linked to a series of includes on MySite that handle the preliminaries, database queries, etc.

I’m now planning on combining all my sites into sort of an encyclopedia that functions something like Wikipedia. I won’t delete the other sites; rather, articles will be available at the original URL and at the new location on MySite/Geopedia.

To get to the point, I’m wondering if it’s time to upgrade to a CMS, like WordPress or Drupal. I have a thousand questions and will have to download and install both and play with them a while. But here’s one question I’d like to ask now…

I have a pretty big series of database tables with articles about a wide variety of topics – mammals, birds, plants, symbols, politics and on and on. If I want to upgrade my sites to WordPress, am I going to have to modify these DB tables, or do I need to let WordPress create new tables, then copy the articles from my original tables to the new database?

I think I need to retain a series of tables, rather than put all my content in one table because of size limits. I tried importing a local table into my online database recently and got a message that it was too big. (I’m using PHP and MySQL, by the way.)

Sorry for the rambling question. I’d just like to have a better idea of what lies ahead for my particular project. Thanks!

P.S. My primary website is http://www.geobop.org (soon to change to http://www.geobop.com), and you can see an example of one of its child sites at http://www.geosymbols.org/World/Arizona.