Moving from Sky LLU to BT Infinity

I'm tempted to move from Sky BB Unlimited to BT Infinity and I was wondering how quick and easy the switching process is?

I've heard it's a right royal pain in the backside to do this particular type of change (LLU back to BT), you'll be without internet for ages and so on - is there any truth to this or is it fairly painless?

I know Sky are just launching their Fibre but I have my reasons.

Thanks

I'm sure BT, if anyone, can do it easily.