You can continue creating your Web Parts as you are now. As long as you are not using any known deprected method in the SharePoint OM then you are fine. There are no breaking changes in Web Part development, more like enhancements that you can take advantage of in SP2010.
My guess is that you have seen the Visual Web Parts and there is no way to convert a "standard" Web Part to a Visual Web Part.
The upgrade path depends on what tools you are using right now. VSeWSS 1.3 projects can be imported to VS2010, WSPBuilder/STSDev requires you to create a new project and import file by file (not that difficult though). And of course you can still use VS2008 and build your Web Parts but you cannot then take advantage of the new VS2010 UI features.