How to Handle Legacy Modernization
First, understand that legacy modernization is not a process you begin as soon as a problem arises. It's not a good sign if the day you begin implementing modernization is the same day you realize your technology is outdated.
Instead, modernization should be an ongoing process. The first step in this process is constantly assessing the mission critical software you rely on in order to preempt when it will become obsolete. Companies that do this well may never miss a beat because of legacy modernization. Instead, they'll be able to update their software as necessary long before it's no longer sufficient for their business needs.
This type of practice should fall under the responsibility of your company's CIO. If you don't have one, it makes sense for the IT department to simply make it an ongoing mission of theirs to audit the technology your company relies on. Any time there are bottlenecks within your company's practices, it can help to investigate them as potential opportunities for legacy modernization.
Another option is to bring in a third party and have them assess where your company may soon be weak because of legacy applications.
While spotting a problem before it arises is important, it's only helpful if you make good use of this information. This means understanding what the problem entails and how you're going to go about addressing it.
Ideally, you can simply use one piece of software to €update€ the one that will soon be lacking. However, many companies aren't so lucky. Understand that sometimes the best solution will simply be moving on to another form of software altogether. While this will be a huge shift for your company, if it's inevitable, you simply have to bite the bullet and get to work on the process. Again, if you begin early, you'll have plenty of time to plan the transition and ensure there aren't any gaps that cause serious problems.
While modernization is rarely an easy task (unless you luck out with a software-fix), the above advice should go a long way in making things more manageable. The main takeaway, though, is that you need to always be vigilante for potential issues and then begin early in addressing them.