'plain_text' filter doesn't exist after D6->D7 upgrade

I just upgraded a site from D6 to D7. Most everything is working well, but there isn't a 'plain_text' filter. After #934050: Change format into string it still has the old name '3' . Many modules expect 'plain_text' to exist, so there are errors in the log 'Missing text format: plain_text.'

Temporary (permanent?) fix is to create the 'plain_text' format. Should the plain_text filter be converted or created in filter.module during upgrade?