What I would do is just take the yeast from primary, wash it, and put it in the back of the fridge. You should have 2 pint jars or so, and those will last quite a while in your fridge, and then step those up (which would be significantly easier than stepping up dregs or a slant) if its been a couple weeks. You will get some drift, but it shouldn't be too bad over 4-5 generations.