You should have the customers upload the local cert bundle now, if possible. Even better, have them move to a system that allows them to trust our roots (X1 and X2) instead of our intermediates.
We plan to switch to the backup intermediates in about a year. But plans don't always work out. We may have to switch to the backup intermediates at literally any time due to a variety of different emergencies, so you and your customers should plan for that possibility.