You can upgrade now if you want to.
Make sure to have the current version of MC12 installed on your system, and all registered and whatnot. Then download and install MC13 from the top of this board. Install it and use it to buy the new version. It will "detect" that you own MC12 and you'll get the upgrade pricing then.
MC13 is still VERY BETA at this point. It works pretty well and is fairly stable, but it certainly wouldn't be considered "production quality" at this point. The main issue isn't that it will be crash-prone or anything (the current build is fairly stable in that regard). This issue is that it is still under very heavy active development, and JRiver may change things that could break your library, make you re-import all your files, re-generate thumbnails, delete all your custom views and start over, chase your dog, scare your kids, or who knows what else.
You can have both MC12 and MC13 installed at the same time, so it is certainly possible to buy the upgrade and keep MC13 installed to allow you to play with it, but use MC12 for your real day-to-day media use. Then, once MC13 has come a bit further along, you can switch over fully.