Actually, before performing upgrade I can suggest you to test v.3 some time. You can also do it on SWsoft website where demo version located. Also, it is better to read documentation to know more aspects about this product.
Sitebuilder v.3 have been changed a lot and not it looks much better and GUI very comfortable.
Upgrade should be done from v2.1.3 and actually it is not difficult one and not painful. In any case, before upgrade it is suggested to create Full backup for previous version of Sitebuilder, so you can always to get back 2.1.3 version.
Yes, that's right. There is no Sitebuilder v.3 for RedHat9. If you are going to migrate also to new server the procedure should be the following:
1. Create backup of Sitebuilder 2.1.3 on your old server.
2. Install 2.1.3 on new server
3. Restore backup from old server
4. Upgrade to new version.
In v2.1.3 there are special backup/restore utilities exists, but in Sitebuilder v.3 there is no backup/restore utilities. They should be available in next v.4 only.