If the spy was successful, why were they recalled?
Or, think about it this way; if you’re spending the time to build a credible cover identity, the agent is able to use that and doesn’t blow it, how do you decide that they have nothing more to offer, and yank them out.
Usually, it’s better to just leave them there, pretending to be a normal person, you know the kind of person who doesn’t wander the streets hiding their face or pretending to be a ninja in their off hours. Gathering information and feeding it back to their handlers.
Pulling them out, when their job is to collect intelligence, is a waste of resources. If you do, you’ll just have to set someone else up to take their place. At that point you’re literally just making work for yourself.
This is especially true if your spy managed to get into a position where they have direct access to classified materials. At that point suddenly disappearing would be more suspicious than quitting their job and making table candles.
Creating a non-official cover identity requires a fair amount of work, so it’s not something you’d want to abandon unless you had reason to believe it was exposed.
If a cover is intact, and was abandoned for some reason, it could be picked up and reused, if the task at hand supports it. A cover your spy has been using will be inherently less suspicious than a new one, because you can point back to their past and say, “yes, that is a real person, and not a fiction that Legends and Missives cooked up last week.”
In this sense, cover identities actually become more valuable as they’re used. A spy creates a paper trail supporting the idea that their cover identity is a real person and not a spy’s masque.
Of course, once someone’s exposed as a spy, that identity is done, and you’re probably going to need to yank them out, and everything they’ve done under that identity is blown. At that point your best bet is to quietly shuffle them off somewhere, give them an official cover (assistant to an ambassador, a diplomatic security adviser, or something else that provides diplomatic immunity, and keeping them as far from anything they used to do.
What you wouldn’t want to do is give a spy a cover identity, recall them, give them a new cover, and then send them out to someplace where they’re likely to encounter people from their earlier cover. That’s a recipe for disaster, with a real risk that both covers would be exposed. You’d use a different spy, or use the same cover (if it applied).
Also, something that is probably worth saying about spies that jump between covers: The most significant work will usually be at the beginning of their careers. Being a spy isn’t like flipping burgers. While you do learn as you go, the more you’ve done, the greater your reputation, the less useful you are in the field. So, when assigning spies for very high profile or high risk assignments, it’s far better to grab recruits who have just finished training than someone with 30 or 40 years of experience. Because with the newbies, other intelligence agencies won’t already have an open file on them. So, while your veteran will have more technical experience and proficiency, the newbie will be more effective, and better able to actually do their job.
This is less of an issue with an agent that simply sits in one place collecting information for years, or sleeping, but if you’re looking at the James Bond, Jason Bourne, Michael Westen, or Robert McCall style spy, those are all characters who are dangerously close (or well past the point) where they can actually contribute anything meaningful in the field.