I wouldn't try and force to change at this time of year. I doubt they'll develop enough to survive the Winter as frogs. May be better to take a chance on them surviving the Winter as tadpoles. I've never found out if Rana Temporaria (The Common Frog) can hibernate in their larval state but I know their are some species that can. This would slow down their metabolic rate enough to survive even if the pond freezes over (which is what adults do). Amphibians are amazing creatures and can survive in some incredibly extreme climates. Post what happens on this site I'll be interested to know.