I'm no professional choir composer/arranger by any means, but my impression from listening to and reading my fair share of choir music is that there's enough overlap between alto and tenor ranges for them to be able to cross over into one anothers' parts in a pinch. It's pretty rare for a composer to write a song with simultaneous use of the altos' upper range and the tenors' lower range, so there probably wouldn't be much need to rewrite anything to make that work.