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I realized that I want/have to update a package (moderncv). The problem is that TeX Live went frozen – so there are no more updates. Is the only thing to do is wait till the release of the next TeX Live?
Is there some way to hack my TeX Live (2012) installation, and once the next version is out revert to the newer release?
You can manually update/install packages into your local TEXMF tree. Usually they are picked up there first so if you put a newer (or older) version of a package there it will be taken instead of the one installed in your distributions (TeX Live, MikTeX, etc.) TEXMF tree.
Under Linux, Unix and similar OSs your local TEXMF is usually simply ~/texmf. For other OSs or if this doesn't work please see Where do I place my own .sty or .cls files, to make them available to all my .tex files? for a way to list this directory. Running texhash ~/temxf after adding or removing files used to be required but modern versions will index the local TEXMF tree at every run if no ls-R file is present there. So either make sure this file isn't there or run the tools (I usually do the latter).
For the actual installation/update procedure see How can I manually install a package but ignore the "on MiKTeX (Windows)" part of the title. A manual install is simply adding the files at the right place and isn't OS depended and the AFAIK same for TeX Live and MikTeX.
After installing the newer version of TeX Live (e.g. 2013 in your case) the packages in the local TEXMF tree must be deleted (again texhash, s.o.). If not your (La)TeX installation will still use them over the potential newer once. I recommend to keep a list of packages you installed this way so you know what files to delete afterwards. Of course if you don't usually have an own TEXMF tree and only created it for this purpose than you can simply remove the whole tree.
tlmgr.