Smart way to manage parallel operations during system transition

When new systems get deployed in any organization, we continue to operate both

new system and old system together for some period. Sometimes, this could be a

year.

Now if we ask end users to enter data on both system, it is going to cause huge

dissatisfaction. This also is clearly a non-value adding activity as customer

does not see any value from the same.

Is there a way out from this?

Here is one actual approach we did and we were very successful.

In this case, we could not let go the old system as the old system was providing

data to many other systems.

So we had to rebuild these interfaces if we would sunset the old system.

Stakeholders did not want to take the risk of multiple interfaces being affected

as those interfaces stood test of time.

So what we did was to build a new interface between new system and old system.

This ensured whatever data being entered in the new system was also available in

the old system as well.

This also allowed us to re-use built rules in the old system and reduced our

development time considerably.

Do share your experience of managing such transitions.