I'm not a financial expert by any means, but a mortgage fixed for the next 30 years at 4.75% would be wonderful when you look at interest rates historically, and would be a substantial saving on your current payment.

I heard that the current interest rates take into account future fluctuations; the base rate can't go much lower, so I would be interested to hear who is speculating that rates will go so low as 3.5%.

In your situation I believe I would be taking the bird in the hand.