Oh man... this is an auto, too, isn't it? You've got a good size pot - no problem with that... the problems come from what you put in it!
Using old soil = asking for problems (bugs, fungus, etc)... Slow release fertilizer = asking for problems since you have no say over what your plant is getting... I don't know anything about plagron light mix but it's GOT to be better than what you've got...
Autos hate being transplanted but that is what I would recommend if you can be VERY gentle with her and not disturb the roots too much... remember, it will have a long tap root. Get it out of that soil... first, autos don't like any nutes for at least a couple of weeks of their life... I grow my autos (which is all I grow) in ProMix because, aside from mycorrhizae which helps roots develop, it has nothing in it and I can control the amount of nutes I give them and when I give it to them.
Be gentle, transplant into better soil, give it a LITTLE water (maybe mixed with some Recharge) and cross your fingers that she hasn't been stunted too badly and will still give you a harvest.
Good luck!