a 115-120l would definately be a comfotable step from a 155 go after a year. i did the exact same thing except i went to my dad's old no nose race board 115l which is about 20-25cm slimmer then anything on the market today, so i had a bit of trouble uphauling on it balance wise. just make sure that you have enough balance to uphaul your sail on it by taking a day or two and just uphauling in a shallow area so you dont get stuck.
once you are confident with that, make sure you can uphahaul in it the usual conditions for your local spots.. ie if you usualy get a fair amount of chop around 1-4ft high, you should realy take the time to learn how to uphaul on a small board in conditions like that, because i got stuck out for about half an hour far from shore because i couldn't uphaul in those kind of conditions on a smaller board then i was used to, so i pulled up the sail and lost my balance everytime until i final got it once and got out of the choppy area...
but it realy doesnt take long to learn if you commit yourself to it, after one season i went from that board to a 88l and can sail it strapped and in the harnais in anything over 15-18 knts. i still love to use the 115l for anything under that, but after 18knts it just starts flying on me with my 6.6 because im light like you, very good board to have though for inconsitent days where you can rig small and then uphaul it if the wind start to die down on you...
more than that it would be hard to help you without knowing what size sails you have and are planing on getting in the near future and what the wind and water conditions are like where you usualy surf... i would recomend trying a couple of boards in the 110-125l area before you buy anything and seing what works for you and what doesnt if you can, that would realy be the bestway of knowing if you are ready to go down to a board that size.. any way you look at it though, if you ceep on windsurfing you will for sure get to a point where you want a board that size soon, so its not much of a question of if you will use it succesfully, more of when you will you use succesfuly, and that should be soon ;p
anyways, let us know what your conditions are like and what sails you have so we can help you out more ;p