Possession is grandfathered, but transfers are prohibited. If you have a magazine that holds more than 10 rounds, you will not only get to keep it, you will have to keep it.
...for now. Keep in mind that this is only a proposal. Negotiations will occur.
A federal blanket ban on transfers may run afoul of the ex post facto clause in Article I of the Constitution and/or the takings clause in the 5A, not to mention the Heller
SCOTUS decision. There is a chance that this provision may prompt the federal courts to immediately block enforcement of the whole law
, a possibility that may prompt even some strong supporters of the legislation to amend this provision away so the baby won't be thrown out with the bathwater.