Textures and Pixel Transfer Operations

I was wondering if there were any way to perform pixel transfer operations (or something similar) on generated and bound textures. Calling glTexImage2D every time the image is rendered is now too expensive for the number of textures I must render. I want to be able to change the Alpha of the texture on the fly, but I want to use glGenTexures and glBindTexture.

Thanks for any help!
asmartin