If you just want to say that it is possible for the code to be replaced by something new/faster/better, you could use the adjective supersedable, which means basically "able to be replaced or made obsolete".1 A more casual, modern way to say the same thing is that the code is not future-proof.2
If you want to talk about how long it will probably be before the code is superseded, then I would use half-life.
The term half-life refers to how long it takes for something to undergo some process. It might be most familiar in relation to radioactive decay or drug metabolization.
In the realm of technology it can be used to refer to how long it will be before some new technology makes a product obsolete.3 So if the code you're talking about will probably be obsolete soon, you could say it has a short half-life.
1 From Dictionary.com:
verb (used with object), superseded, superseding.
- to replace in power, authority, effectiveness, acceptance, use,
etc., as by another person or thing.
- to set aside or cause to be set aside as void, useless, or obsolete, usually in favor of something mentioned; make obsolete:
They superseded the old statute with a new one.
2 From Techopedia:
Definition - What does Future Proof mean?
Future proof is a buzzword that describes a product, service or
technological system that will not need to be significantly updated as
technology advances. In reality, very few things are truly future
proof. In any field that depends heavily on technology, a regular
cycle of replacing and updating appears to be the norm.
3 For example:
[W]hat is the half life of your technology devices these days? By that
I mean, at what point are you already starting to think about
upgrading your devices — from the moment you purchased the last one?
Half-life is the moment when you could continue to use your device but
it is either so far behind the functionality of other devices or —
because of performance issues — it is increasingly unusable. Both
points used to take three to five years, but are now more likely to be
only one or two years.
—"Why is the half-life of technology getting shorter?" Global Telecoms Business, 2012.