[walshemj]

One large company (170k employees) I worked at switched from an in house payroll system to an external month.

Second month was a disaster in the end they had to physically cut the tape on the last remaining partially destroyed copy - about 20% got paid on time.

[fludlight]

Switching a large enterprise software system is almost as time consuming, expensive, and error prone as developing a replacement in-house. You would have experienced substantial problems even if it was in-house -> in-house-rewrite or external->in-house or external-a -> external-b.