Sometimes, but not always,
MDE will allow future bookings of an attraction that is then down. I think it allows it more often than it doesn't. It may depend on how long they expect the attraction to be down for. If the attraction remains down when your return time comes, it should automatically convert the LL for that attraction to an anytime LL (also known as an MEP and by other terms). Sometimes, it will convert it even before the return time begins. If that was your most-recently-booked LL attraction, you will also then be allowed to book another LL right away. The anytime LL can always be used for the attraction that went down, as well as other attractions. It will tell you which ones it can be used for. It won't be all attractions though. Booking another LL attraction will not remove your anytime LL. Tapping into the
lightning lane for another ride that accepts that anytime LL, and for which you do not have a LL for that ride would, however, use up the anytime LL, So, the potential problem isn't with
booking another LL; it's with
tapping into one. If you are careful about tapping in to the lightning lane only for those rides that you have a LL for that ride, you should be able to hold onto your anytime LL until you can use it for the ride of your choice.
Edit: I'm using LL to mean Genie+ lightning lane
reservation, not the lightning lane itself.
You do not have to have tapped in to the ride that went down to be given an anytime LL.