Simple enough. But is there a reason to replace the convertor when there wasn't a hard failure?
The reason is that you don't want debris that's in the old converter causing a failure of the new trans. And yes, there IS debris in it.
I wouldn't mess with it then. Just flush the cooler really good. If your converter is locking up fine leave it in there. Like I said if it goes your only out of a little time to switch them and fluid.
This is an excellent plan to cause you to rebuild the trans a second time. The second time you do it, plan to change the converter AND the coolers so that you don't have to do it a third time.
The coolers and converter both have debris from the old trans. If the old trans was run in the truck it has created debris. Some if it gets stuck in the converter, some of it in the coolers, and some in the filter.
There is NO effective way to flush a converter or a cooler. The converter could be cut open, cleaned and inspected, and then rewelded. Then it would be safe to reuse it. The coolers should be scrapped unless you're willing to take the chance that something could find it's way out of the cooler and cause an issue with the new trans, up to a failure.