Old does not automatically mean obsolete
A decades-old system may still process payments reliably. Age alone is not the issue: unsupported technology, few people who understand it, fragile connections to other systems, or missing tests can make changes risky.
New software can become legacy quickly if no one understands or maintains it.
Ways to change it
Teams can document and stabilise a legacy system, replace parts gradually, move it onto technology that still receives support, or rebuild it. A full rewrite is risky because the existing system may contain important business rules that were never documented. Teams first need to understand what it does and which changes are worth making.