If you're treating the numbers as versions you're probably not using CSS, but CSS2.1. CSS2.1 and CSS3 are 'levels' of CSS. The W3C levels like this:
Cascading Style Sheets does not have versions in the traditional sense; instead it has levels. Each level of CSS builds on the previous, refining definitions and adding features. The feature set of each higher level is a superset of any lower level, and the behavior allowed for a given feature in a higher level is a subset of that allowed in the lower levels. A user agent conforming to a higher level of CSS is thus also conformant to all lower levels.
Note that the conformance to a level is a thing is for the user agents (browsers), not the authors. So to answer your explicit question: you don't have to do anything to migrate from CSS2.1 to CSS3, browsers should continue to support all your current CSS.
If you want to take advantage of new features available at CSS Level 3 then you should first ensure that your users have browsers which support those features. A good reference for that is Can I Use. Note also that good progress has already been made on some CSS modules at CSS Level 4.