Thankfully, there are some nice utilities that make conversion easy.
- RMVX Characterset Converter (RMVX): This awesome utility makes it possible to convert RMXP character sets to RMVX format, which can then be saved as BMP, JPG or PNG formats. You can even convert those old RM2K/3 charsets to RMVX formats. It can be downloaded here.
- RM2K/3-To-RMXP Resource Converter (RMXP): As the title suggests, this utility converts those old RPG Maker 2000/3 resources to RMXP. I've used this on numerous occasions to make games compatible with RMXP. Note: When using this program, be sure to select the GAME folder for the resources to convert, NOT the resource folder itself. The utility can be downloaded from Kobra's Realm.
- Charas.EX (RM2K/3): This utility allows you to create charsets from scratch or modify existing charsets. Although the utility is specifically for RM2K/3, charsets can be converted using the two utilities above. The program can be downloaded from Kobra's Realm or charsets can be developed online at Charas Project.
- RMXP Resource Wizard (RMXP): This program, in my opinion, is the definitive RM2K/3 to RMXP converter. With it, you can convert any RM2K/3 resource into a compatible RMXP format. You can download it from RPG Crisis (you will need a membership to access the file).
These are the only good resource converters I know about, but if I find (and test) any more I'll add them.