UVB bulb lifetime rays?

I use 10.0 uvb bulbs and replace them every 5-6 months, I am thinking about getting some chameleons which need 2.0/5.0 lighting, Could I use my used 10.0 bulbs? A bit of a strange question but my theory is it is not giving off full UVB but it is still giving of some which is the equivalent to 2.0/5.0?