Do you need to keep the same speed? do you need to keep the video sync with the audio?
In After Effects, you could interpret your 12fps source footage to your master timeline frame rate and see if it suits your needs. As suggested you could also use Time Warp and set to best quality (optical flow).
If you are considering using an external plug-in, I would recommend Revision FX Twixtor which will allow you to conform from 12fps to whatever frame rate is your master.
In any case quality of the conversion will depend on the quality of the original footage and the type of motion involved inside of your frame.
David Baud
Post & VFX
KOSMOS PRODUCTIONS
Denver – Paris
http://www.kosmos-productions.com