Your filesystem is probably corrupted enough that it now boots in read only mode.
There is a known kernel issue affecting compatibility with some SD cards on the Pi 2 that leads to progressive file system corruption. As a quick fix you can try to run fsck on it, as described here:
Long term fix is that you will need to do a fresh install once RC3 is released as RC3 includes a fix for this issue. For more background see:
If you have a different make and model card that is not affected by this issue then yes, a fresh install on a different model of card would also solve the issue, and after RC3 you would be able to safely use the original card again.
It’s no use imaging your existing card and transferring it to the new one as you would copy all the corruption across. A fresh install would be required.
I haven’t had any corruption problems on my Pi 2 using the 8GB noobs card.