Hey DuTaz,I think that soil will be allright,although I don`t have personal experience with it.
Shamrock, Natures Own Multi-Purpose Compost
20 litres £2.98

This had a very silky feel, and is clean, safe, pleasant and easy to use. It contains all the food and trace elements plants need for strong, healthy growth. This compost claims to retain the right balance of moisture, air and nutrients around the plant’s roots. It is based on coconut fibre – a natural renewable resource which is a by-product of the coconut industry of Sri Lanka.

Ya didn`t specify what size pots you have them in now.If your going to transplant,nows as good a time as any.If your leaves are turning yellow,you probably need to start fertilizing.Sounds like you have a nitrogen deficiency.These soiless mixs that have the nutes already in,aren`t going to be able to feed a high energy plant like marijuana.Start nutes as soon as possible.What are you using for nutrients?You shouldn`t need a booster if you have a good nutrient regimen.If you haven`t bought nutes yet,I can recommend several good nute systems.Hope this was of help.
Peace