mold on aluminum window frames

I have severe mold on my bathroom window aluminum window frames. I ordered new windows that come with vinyl frams and instead of removing the old frames they plan to do an overlay, my husband and I are worried that if the old frames are not removed that this will cause major problems later on. Does anyone know if this is ok to do or shoule I insist that they remove the old frames as promised when they gave us the price quote?

Any help on this will be much appreciated.

Thanks

Sweetsal