If you have a 64 bit PC and OS, it should be little more than a recompile.
A number of comments have claimed this: recompile with 64-bit time_t and the problem goes away. Unfortunately, for many apps it's not quite as simple as that.
Certainly, for those apps which only deal with time information internally, and in a transient fashion, this will be sufficient to eliminate the problem.
However any program which persists UNIX timestamps in files, or sends UNIX timestamps as part of a networking protocol, or basically anything which sends the data structure outside the application is still going to require work on how to handle the migration.
What happens if your app is required to talk on the network? If there are few enough machines involved, then sure, you can upgrade all of them at once, but if it's a large or distributed operation, there needs to be a transition plan. How will older clients and newer clients interoperate?
If data is saved, how will the recompiled application interpret old files? Does it need a way to distinguish them? Can old data be automatically converted? Are there cases where old data may be compromised? (e.g. those 30 year mortgages probably have the wrong end-date stored...) How will we handle those cases?
What about situations where time information is used to prime other calculations - is it okay to affect those other calculations? Serial numbers, PRNGs, UUID generation, etc - the situation has to be assessed. Many cases might be fine, but you can't make a blanket statement for all cases, so each calculation must be vetted.
In the end, the result is that there is work to do. Much of it will be easy to fix, but the assessment still needs to be done.