I have never found it necessary to uninstall a previous version of Opera when I'm installing a new version, and I've always installed the version Opera provides and have not waited for a VL package. Opera creates a new folder for the new version in /usr/lib. It's true that some files from the old version will be overwritten at other places on the system (/usr/share/opera, for example, the executable in /usr/bin, some files in /etc).
I installed Opera 10 on two computers today (VL6 Standard and VL6 Light). I'll put it on the third computer either later today or tomorrow. I downloaded the version for Slackware 12.1 as nightflier mentioned. So far everything has worked very well--even better than Opera 9.64.