I hope the direct guys don't mind me quoting them but this is what I was told when I asked the same question:
There may be some downtime on the change but it will be minimal. Essentially you would have to setup for the change of phone service first and then inform us of the date that your regular phone line will be dropped. We would then request the activation of the dry loop for the next business day. We need at least 5 business days notice in order to make this request.
There is a possibility that a tech may be required to activate the dry loop, if that is the case it will be an 8-5 time frame.
The change to dry loop is treated as a whole new order so there will be an activation fee of $69.99 plus the first month of service that will be due when the order is placed. Then any time remaining from your regular DSL will be credited back onto your next statement with the dry loop.
The pricing for the dry loop itself varies depending on the area. If I have the correct address here you would be looking at an additional $9.10 + tax on top of what you are currently paying monthly.
If you're cool to do some internal rewiring then Koreyb's idea is a good one. There's only 6/800 DSL in my area. When all considered I ended up paying a little extra upfront and switched to cable instead. Monthly I now pay within a couple of dollars of the dryloop DSL6 cost but with better speeds cable provides. Cable install process went flawlessly and on time.