you can switch up to 18/6, but you are going to put the plant under stress telling it to switch to the veg cycle...increasing grow times, then you have to switch it back to 12/12 and wait again for the flowering period to restart...If you want your plants to finish in the next 1.5-2months then keep it at 12/12, yes you will have a lower yield, but in the long run is it really worth extending your growing period out to 4 months for probably a 20-25% gain in yield due to stress?
I'd just finish this cycle out, then next go around go 18/6 for 2-3 weeks from the start, then flip to 12/12