HELP NEEDED! with OLD/NEW LAMPS

Hello respected growers at RIU please can you help me with this dilemna that i amin. I have bought 3 X 1000w lighting units ballasts ,reflectors and lamps secondhand all for 240 pound. BUT the HPS lamps i got are pretty old, the guy said he has done 4 grows with them bulbs, will this be a problem. shall i spend another 150 pound buying 3X brand new 1000w Dual spectrum lamps (nearly as much as i payed for everything to begin with) or shall i just use the old HPS lamps. Why i ask is because my friend says that if the lamps are old (more than 3 grows), the end total yield is significantly reduced, 30-40% less bud! IS THIS ACTUALLY TRUE? it sounds wrong to me, its the same 1000w lamp no matter how old it is becoming, if it works (15000/50000 hours still left)it should be just as powerfull as normal or not??????????
PLEASE SOMEBODY HELP ME IF THE YIELD IS SERIUOSLY REDUCED THAN IT WILL BE WORTH IT BUYING NEW LAMPS BUT IF THERE IS NO DIFFERENCE THAN THERES NO POINT IN SPENDING ANOTHER 150 POUND MAKING MY TOTAL SPEND 390 FOR 3X 1000W LIGHTING UNITS WHICH IS NOT WORTH IT

use them until they stop working. When the bulbs are nearing the end of their life you will notice a change in the light given off them (you can find a video of one nearing the end on youtube), it is this that effects the yield. It's unlikely that all three lights would die at the same time, however I would perhaps buy one replacement bulb for emergency. I think you should always have one replacement bulb handy just in case.
So basically buy one bulb, keep it until one of the old bulbs dies or is near dying, then replace it with your stored fresh new bulb, then you buy another spare bulb for when the 2nd old bulb ends and so on. This way will save you spending 100 odd bucks on 3 bulbs at once, and you will always have one spare replacement bulb at hand.