You can update your site's content as often as you want, and as far as using a different CMS each time you update a website goes, you cannot install 2 CMS products in the same URL (We'll use Wordpress and Joomla for this example ) -
You can use WordPress and Joomla on your hosting account, but not on the exact same URL, unless you use the Wordpress in Joomla extension. You can also use WordPress and Joomla on the same hosting account, but in different subdirectories. That is no problem. For example, you could install Joomla on http://sample_domain/cms while you have WordPress installed on http://sample_domain/blog. You would need to reference each URL in the Joomla website or the WordPress URL
Both Wordpress and Joomla generate the URLs for their pages, which one of the reasons why you cannot have them installed together on the same exact URL. For more information, check out our educational channel for Wordpress over here. You can find more on Joomla here: Joomla 2.5 and Joomla 3.1.
I hope that helps to give you the answers you seek. Please don't hesitate to ask more questions or comment or comment further.