Education: When you learn new things, does this make you forget older things?

I am doing a job with a certain type of computer technology - call it technology A. I have the chance to learn and start working with technology B. But I am worried that if I do this I might forget some of my skills in A which will devalue me in the contract marketplace. Is this a valid concern? Obviously I don't want to learn a new technology and find it devalues me rather than improves me.