When a tool the business depends on has become slow, fragile or unloved, the instinct is to replace it all at once on a set date. For a system people use every day, that bet is bigger than it looks: the switch-over weekend becomes the only chance to find out what the old system quietly handled.
Take one slice first
A safer approach is to move one part of the work to the new tool while the old one keeps doing the rest. Pick a slice that is small, self-contained and used by people who are willing to give feedback. Run both for a while. Fix what the real work reveals. Then take the next slice.
What it costs
Running two systems side by side takes more effort than a clean cut, mainly in keeping data consistent between them. It also takes longer on the calendar. In exchange you can stop at any point without losing a day of business, which is usually worth far more than the extra effort.
When a clean cut is fine
If the system is not business-critical, or nobody would notice a day without it, a straightforward replacement is reasonable. The gradual approach is for the tools you cannot afford to be without.
As with all general advice, the right path depends on your system, your team and how much disruption you can absorb. The first step is understanding what the current system actually does, including the parts nobody wrote down.