Cleaning Attached Sofa Pillows?

I've had a sofa/bed for over 20 years. It's still in very good shape, but the color is a little outdated. I'm shopping for slipcovers, but have a dilemma with the pillows. They're large, and there's a lot of them (they're part of the backing of the sofa and love seat).

How do I clean them so I can have new covers made for them. I don't want to clean just the outsides, which I've done, but the insides must be yucky after all these years! Or would it be better to hire a seamstress and start all over with new pillows? Your thoughts would be most appreciated! Thanks!

Casey from Plano, TX